"I've heard it is best to nail up a backer board into which to nail the crown."

It is a lot easier to find studs and get the backer board solidly anchored before putting up the more expensive crown.

A few extra nails in the backer to hit the studs gets covered by the actual trim.

One of the easiest 'tricks' to coping is to miter the molding, then cut away the wast to the miter line.

Thanks Brickeyee, I was going to miter on my chopsaw. But I never end up with a really tight fit. Does using painter's caulk and painting give the best solution to 'fixing' the open miter? Or use plastic wood and sand down?

First, buy one of the books available at home improvement stores that covers crown molding installation. That book will answer most of your questions.

Second, there is a big difference between installing stained and painted molding. Mistakes can be covered when painted. Not so with stained.

third, coping is done to eliminate the necessity to fill an open or ill fitting joint. Coping also minimized the gaps from seasonal wood movement due to humidity and moisture. That means if you have a gap in a coped joint, it is done improperly.

The joint you described about joining two pieces in a run is called a 'scarf' joint. To hide that joint, there are a few tricks. First, there are definitions. Open and Closed sections of the scarf. Open is when you can see the material of the crown and the angle. Closed is where only the profile of the crown is visible. To help hide the scarf, always create the joint so the closed end is overlapping the open end as the eye sees the joint.

That allows the eye to look past the line of the joint instead of looking into the joint.

Now, the problem with your miter saw and the scarf joints. To minimize that problem, simply plan the cuts. Set the saw for a cut. To make the matching miter, simply set the stock on the side opposite the first cut---do not change the saw. You can make those cuts with the stock flat on the table(tilting the saw to 45 degrees) or standing up(if the saw is a 12" blade model with the saw swiveled to 45 degrees).

A new sharp saw blade will be a help.

Also, when I did my scarf joints, before putting the crown up, I glued the joint and glued a about a 10" long, 1" wide, piece of 1/4" thick wood to the back of the crown across the joint to help strengthen it. Then I installed the whole length of crown. I'm hard pressed to find the joints now, but this was painted crown - like handymac says, MUCH EASIER.

Thanks guys. I have one very long wall to do: ~25'. Would a good plan be to do this moulding first? Construct it using the scarf joint, put it up (with 2 helpers)and then add on the other pieces using coping to make the inside corners? Originally I was thinking of joining the two pieces on the ceiling but I like the backer piece idea.

On a mulit-piece run, always place the coped side first, so the length of the continuation is non-critical (the butt against the wall is covered by the next coped piece). Don't work into the corners, always work toward an "open" joint; except for the last piece, which will have two critical cuts, better if one can be a short outside miter.
To learn how your molding meets the ceiling and wall, cut a pair of short test pieces, so you can learn how and where they interface with each other. Mark the points of contact on the ceiling and wall, and work to them, that ensures a properly aligned cope. You can leave the last 16" of an working end un-nailed so it's wall/ceil. relationship can be adjusted by tapping up or down as needed to tighten the cope and bring all into perfect alignment. It doesn't take much to get a perfect cope, but if the molding is against the corner at the wrong angle it will look like h311.
Casey

Making the initial cut for a coped joint on a chop saw works fine.

You then cut away all the new end grain using the miter line on the face of the molding.

You do need to watch the molding pattern though.
Not all molding can be coped.