I have heavy clay soil and over the last two years have incorporated A LOT of manure (eqivalent to a 6"-8" compact layer over the whole plot in total).
I also increased the depth of the shallow soil by digging out paths, putting drainage pipes into the base, filling up the 18" deep trenches with scalpings, then sharp sand, and finally slabs, and throwing the soil over the rest of the beds. This did a lot to improve the drainage (ie I improved the drainage and raised the height of the soil).
I also over ordered on sharp sand (and extra 6 cubic meters over and above what was needed for the paths for a 105 sqm plot).
Over the last year I used the sand as a 2"-4" mulch for beans, peas, toms, peppers, root veg. Mixed it into the bottom of potato trenches too. Most stuff in fact. In fact the only place that it wasn't used was on the seed bed. Then forking it into the top spit of soil as the plants were harvested.
I now have a fertile, moist, much lighter, free-draining soil.
I asked for some advice with respect to this and the consensus was that it was a BAD THING to consider because so much sand could contain salts that would damage the crop. I chose to ignore this advice as they would probably be washed out after a year anyway. I might loose some crops, but it's all experience!
The sand mulch kept the slugs at bay. Where forked in the drainage is brilliant. No problems with yields either. My experience is that sharp sand is cheap(ish) and will do the job well, but that depending on your soil you might need quite a lot of it. I wouldn't incorporate it into the ground during winter although there's not reason why you shouldn't - I'd dig in loads of manure etc. I'd go down the sand mulch route because there was no weeding or slugs on those crops.
Shop around, haggle. I paid £28 per cubic meter delivered, but it did take a bit of negotiation and I probably got it close to trade price.
SS