Just from my personal experience with our soil -- which I believe was a "clay loam" -- adding sand, although I do realize this isn't necessarily a recommended amendment, and then tilling in a LOT of organic material (our grass clippings, mainly) for a number of years, has totally changed the soil for the better. Originally there was barely a worm to be found, now a shovelful of soil reveals multiple worms (and, again, the soil has excellent drainage). This year was probably the first year (after 20+ years of having this garden) that I didn't till -- I normally till in the fall and we had a LOT of rain so it was too wet and I figured I'd just till in the spring; then we had a LOT of rain in the spring and I just said the heck with it and tidied the garden up with a hoe and rake -- most of the clippings, leaves, etc. that we had piled on in the fall, probably about 2' deep were decomposed except for a couple of inches of rough stuff left on top -- and planted. Where I was planting something like carrots I loosened the soil with my garden fork. Not sure right now if I actually ever need to till again, but I do think it's necessary in the earlier years of improving your soil. Just my opinion...
As a side note, our lawn doesn't look as good as a result of all that organic material being removed from it... but I don't care that much about that!
