Hi floral,
The more info you can give us the more accurate the responses will be.
1. What is causing the shade in your back yard, buildings or trees? If it is trees what type are they?
2. How many hours of sun (if any) and what time of day.
3. What are the soil conditions, are they wet or dry?
4. Do you know the soils ph?
5. How large is the area?
6. If you can photos are always helpful.

Beside the answers to all questions Kt asked we also need to know what kind of look you are planning to achieve: 'natural looking' woodlands or more refined 'garden' border? That will basically dictate the choice of plants and their placements in relations to each other.
P.S. It's my strong opinion (after numerous trials) that mophead hydrangeas do not belong to woodland borders while lacecaps could be used very effectively in both, 'woodland' and 'civilized' borders.

Of course I should have given more info...sorry about that!
The back of my back yard is shaded by a combination of surrounding structures and trees. As you view the area from the rear deck off my house you would see my garage on the left(which is East), a neighbor's garage at the back (South) and a neighbor's shed on the right (West). All of the buildings are currently painted white. (if I was not at work I would be able to post a picture - sorry!) The existing trees include two tall pines in the southeast corner of the yard and a large maple in front of the shed on the west facing side. The only real sun travels across the spot for just a couple of hours in the late afternoon - it just sneaks in under the shadow of the maple. Hopefully this is making sense so far....
I am in the process of amending the soil but it is currently rather dry. I have not tested the pH. My assumption is that it would be on the acid side due to the neighboring pines. The area I am hoping to fill is no more than 20-25 feet in length. I'll make the bed as deep as I need to in order to achieve a good result. Though I have a few trees, my setting is definately not woodland. The gardens I have begun in the front of my house are decidely cottagey in style. What I'm going for in the back is a slightly more planned, but still very natural look.
I hope I have covered the most pertinent bits of info but by all means let me know if you have any other questions. Right now what I have is a blank slate so I am excited to hear any suggestions. Thanks again!

putting aside style for the moment, hydrangeas definetly do not like it dry.

I have not had first hand experience, but everyone seems to complain about planting conditions under (near?) large maple trees. Not just thirsty plants, but most any plants. Successful plants are limited. Pachasandra comes to mind, but don't go there. Hate the stuff... am yanking mine out.

nice description of the yard.

Do you intend to create a screen from the neighbors structures? Evergreens would lend itself to that as well as provide winter interest. However, for shady areas, taller evergreens are limited.

In my dry (but irrigated occasionally) shade mixed border I have pieris, leucothoes, hollies, yew, dogwood, viburnum, hydrangea paniculata 'Limelight', The non-mopheads do not seem as fussy about water. Also have russian cypress, heuchera, hosta, and that's as far as I can go on memory!

Am looking forward to see what the experts chime in with.

Wendy, cherry laurels could take dry(ish) shade and still be in 8-10'range.
You could grow many plants under large maples, but two conditions has to be met: maples has to be very large and area must be irrigated.
The whole idea is based on a fact that than closer to the trunk of the LARGE maple you go than less young and active feeder roots you'll find. Therefore, you have to build raised bed around the trunk of the tree (leave at least 1' space between new soil and a trunk) and plant shallow rooted plants. Bed has to be raised enough to accomodate rootballs of those plants without digging into the maple's roots.
Old gardener taught me this trick and I have all kinds of ericaceous plants (kalmias, azaleas, rhododendrons, pierices) plus jap.maple, skimmias, leucothoes, hostas, ferns and heucheras happily growing in a such bed for over 5 years. Kerrias, clethras and astilbes didn't make it, though.

Oh yea, forgot to mention, climbing hydrangea was put at the base of that maple and now 20' up the trunk.
Blooming first time this year :-))

Thank you all for the great ideas! I have been seriously considering creating a raised bed and after all your input I think I will go ahead with that plan. It is a challenging spot so I suppose I really should so anything I can to set myself up for success rather than heartbreak!
wendyb - I do love the limelight hydrangeas (I have 2 towards the side/front of my house) I don't know why I hadn't thought of using them in the back border, too. Knowing that you have had good luck with them in drier conditions is making me feel that they are the way to go here. What variety of hollies and viburnums do you have? Do you think one or the other works better with the limelight hydrangea?
ego45 - Thank you for convincing me to do that raised bed that I've been waffling on...Honestly I know it's the right thing to do but was trying to talk myself out of it to save some work. How lazy of me! I know the maple roots run under my desired planting area, but it's not really close to the tree trunk (about 10 feet away). Would you have any guess as to how high I should build up the bed or is it just a matter of finding how deep the maple's roots are and building up to accommodate the roots of the new plantings? What is your raised bed edged with?
runktrun - I know what you're saying about planting too many shrub varieties in a small space and I really am trying to control myself. I was thinking of having a few shrubs each of just 2 varieties and then maybe adding just one taller shrub or small tree for a spot of contrast. Then I would underplant with perennials. Do you think that sounds appropriate? ( I will hopefully have a good spot for my hellebore seedlings from the plant swap - thanks again!)

Thank you all again for the many excellent suggestions and tips. Some of the things I had not considered were pieres, skimmia, viburnum and Japanese Maple. Any suggestions on specific cultivars is appreciated, too.

Re: how high the raised bed should be?
As high as you could afford to do it, but 6" I would say would be a bare minimum. You don't have to find the maple roots, they are right on a surface. The whole idea is NOT dig and disturb them, disturbed/removed roots will reproduce very agressively, but rather create a headstart conditions for your new plants before the maple roots will reach surface again. And they will, don't fool yourself by hoping otherwise. But by then your plants will be established and with help of irrigation will be able to stand against the maple.

Re: edging.
Rocks. I have plenty of them.
Re: jap.maples.
I'd recommend either 'Tamukeyama' or 'Inaba Shidare' since both of them are relatively fast growing and readily available from HD/Lowes. In considerable shade 'Inaba Shidare' will retain dark burgundy spring color much better and longer than 'Tamukeyama', but by the middle of the season may become dark green before turning red again in a fall. Both have a weeping habit.
However if you want to brighten the spot, "Butterfly' with its creamy-white overall appearance should work better. This one have a very well defined upright vase shape and would allow some planting underneath of it. There are several different clones (varying in a level of variegation) on a market sold under this name, so this one should be purchased only when is fully leaf-out to know what you are buying.

Agree the JM is an excellent suggestion. I have one also in my bed and it adds a real pop of color, which is important in a shade bed. I have inabe shidare. another fave is Tamukeyama. Both hold their color well season long.

I have viburnum mariessi 'Dawn' and it doesn't bloom very well there yet, but its a nice plant overall. viburnums are just so easy-care. I have viburnum dentatum in similar spots and they bloom fine.

The hollies I have are Blue Princess and Honey Maid. I bought (invested!) in 3 large Blue Princesses (4' or so) to give me some instant screening from the road. They have been great. You may have other choices though. I am in a colder spot. Blue Princess is very hardy. Honey Maid had some damage, but it adds a pop of yellow especially in winter.

My pieris in that bed is a no-name that came with the property. However, last year I bought 'Silver Flame' for another spot. I LOVE it. Awesome pink long-lasting new foliage coloring and variegated otherwise.

ditto ditto ditto spacing and planning. My bed was created about 4 years ago. This year I extended it and had to transplant 14 plants due to overcrowding. What a pain! Its possible at the time of planting I told myself, "well, I can move it later" or "I'll just put this here for now temporarily until things fill in". But who the heck remembers that!!! All I know is I was going nuts last year watching everything grow into everything else and not having anyplace to walk.

I also added a yew, taxus capitata for some add'l screening but the deer had something to say about that :-( I don't usually have deer trouble, but last winter they were especially hungry and found all sorts of goodies.

Oh, yeah that reminds me. I also have chamaecyparis 'Goldmop' in there. It isn't so gold in part shade of course, but that suits me fine. Its a nice chartreuse color.

And another great perennial for sucha spot has been Jacob's Ladder - Polemonium (sp) 'Stairway to Heaven'. 'Touch of Class' would be similar.

Let us know what you come up with.

Again, thank you so much for the excellent information. I'm really starting to feel enthusiastic about transforming this rather unsightly area into something special. I'm a little concerned that the limelight hydrangeas may get a bit too much shade to flower well, but I think if that were to happen I could limb up the maple a bit to let in some more light. I do have some dappled sun at various times of the day so it might be enough as is.
I stopped by a garden center to get info on a delivery of screened loam/compost so hopefully I'll have my raised bed in the works soon. I also have plans to go shrub shopping tomorrow at a plant nursery near my mom (in CT) that is having a big sale. I'm not holding my breath that I'll find everything I really want, but it's certainly worth a try and would free up my budget for more dirt and irrigation! I am really looking forward to getting a Japanese maple. The more I think about it the more "right" it seems that I should have one in that spot (and for some reason it had really never occurred to me) - I looked up all the varieties suggested and love them all so I'll just see what's available locally and what looks healthy. wendyb, I will be sure to update you on my final choices - and thanks for sharing your cautionary tale about overplanting! I may already be in for that type of headache with my front flower garden. oh well, I will really try to do better with this planting area!

George, Wow! I was composing my response when you replied and didn't even see yours til just now. We both recommended the same JMs!! That is amazing... I wonder if you recommended them to me 4-5 years ago? At the time HD/Lowe's didn't have much other than Bloodgood and Crimson Queen! blech...

Speaking of roots, (floral, can I borrow this thread?) I have a question about pine tree roots. Do you know pine tree roots as well as maples?

The back of my bed is about 20' away from a bunch of 75-100' pine trees. There are also assorted oaks and hickories and smaller maples in the vicinity. When my bed was created, I had the landscaper crew dig the big holes and plant most of the plants. The digging gets mighty tough in that area for me. Anyhow, they had to cut through lotsa roots in the process. We decided that any roots that belonged to the large trees would recover. They spread a few inches of compost over the entire area that was "lasagna'd" the fall before. And they did amend the holes as well.

Well, now 3-4 large contiguous pines outside of the bed area are dying. Its been going on a couple of years and now they are pretty much toast. They are not on my property actually. they belong to the town, who owns a strip of wooded land between me and my neighbor. Do you think all that "root pruning" could have caused such a disaster? These dying trees represent about 10% of all trees that abut my bed.

Wendy, I don't believe that such digging/root pruning as you described it could of done ANY damage to 75-100' tall pines. I'm sure there is some other, more natural cause.
And since you mentioned oaks. You could dig and plant anything you wish to in their root zone, even directly next to the trunk, for as long as you are not raising the soil level. However, even slight raise, say 3-4", may kill mature 100 years old oak in just 2-3 years. Therefore creation of raised beds around the oaks is not advisible if not prohibited.

that's good info. May be sacrilege, but I wouldn't mind if I lost any of the oaks. Their leaves come down so late and require all that shredding. a real pain.

But, speaking of raising the soil level, what about grass clippings and yard debris at the base of these pines? Not directly at the crown of course. Just a few feet away though on one side. I would think the feeder roots are nicely into my bed and throughout the area and are getting all the water & oxygen they need.