In my raised beds, the best I have had is a mixture of approximately 50% peat moss and 50% composted sheep manure. (Sheep manure being the advice of an Italian neighbour over cattle manure) Each year I threw a small amount of homemade compost to top them up. The manure disappears very quickly into the soil, so the peat moss adds longevity to the levels. It takes about 2 years for peat moss to disappear into my clay soil. Peat moss will also hold soil nutrients and keep them from leaching away. In the fall, I turned some leaves into it. Some people say just to leave them on top, but that attempt made all the leaves blow away with the first wind. I never noticed any negative effects on my plants. In the spring, the level would go down somewhat and I would turn over the bed digging a bit into the clay soil beneath. (I started them as sort of a lazy lasagna bed) I'd top it up with whatever composted or semi composted material I had and finish off with more compost or peat moss. By now, I'm sure the soil in the bed is a good part of the natural clay soil mixed in with the added components. I get great results every year, more produce than I can deal with.

10CY sounds about right as a soil amendment. As fertilizer, you need a lot less, perhaps 0.5-0.8 CY. Bought at a garden center, it will cost a fortune. But - you don't need to add it in one year - you can dilute the manure cost with some heavy wood chip mulch (plus some nitrogen). The wood chips will eventually compost, and they should be free of charge if you contact a tree company and ask them to bring you a load. - you can further dilute it by collecting bags of dead leaves and yard clippings in your neighborhood. Discard only the bags with grass clippings treated with pesticides. All the mulch that you pile this year is compost you don't have to buy next year. You still have to get 10CY of organic material eventually. See if some dairy or horse stable will deliver manure to you, and load up on dead leaves this october.

I'm also a devotee of the concepts behind the double-digging method. The important aspects, IMHO, are tilling as deep as you can when you originally make new beds. Then you establish regular beds and paths, and don't walk on the beds if you possibly can. Amend heavily in the fall, and then as Michael said, leave soil improvement to the experts. You can work the amendments into the soil with a rake or a garden fork a couple inches if you want to, but you don't have to. Lots of folks till every year, and that works fine. But I'm worried my "micro herd" of worms, soil bacteria and fungi, etc. will be too disrupted. Besides, I don't know anyone with a tiller, so all my bed prep is done by hand. (Thankfully, I have the help of an agreeable hubby and sometimes of a strong teenaged daughter.) This year, I was able to plant some things in my oldest beds with my hands only if I wanted to. Not tomatoes that I plant deep, but peppers, lettuce, etc. As of this year, the tilth in those beds is amazing. I've been working those beds for about 5-6 years. Good luck with your new garden! I'm curious if it's windy on the top of the hill?

I found a source of well composted horse manure. The horse owner mixes kitchen waste etc and turns the pile during the winter. This year I was late getting it so I planted sugar snap peas and spinach without adding anything to one bed. Things grew fine but the spinach I later planted in another bed after the compost was added was much bigger and healthier. Taught me a good lesson. I added fresh steer manure to one bed one year and did not like it. It went on too thick and hardened up. I still planted in the bed the following spring but made planting holes for winter squash. Even over the winter the manure didn't break down very well.