Well Harvey you have come to the right place, we have a tonne of threads on ammending poor soil, use the insite search engine to read up.
For now, I'll say that sand will help a bit but, what really improves both clay and sand based soils is adding organic matter. Manure is considered organic matter but, in composting soils is considered a "GREEN" which means that it has a Carbon : Nitrogen ratio of about 30:1 or less.
What you want is a balanced addition of greens and "Browns" a brown is something with a higher amount of Carbon like leaves, newspaper (black and white articles only).
I personally like to add leaves as apposed to newspaper but, do sometime lay a few sheets thick pile of newspaper over weeds that I have already mowed over (and left the clipping on the ground) and then add a green followed by a layer of leaves and so on.
Vary your layers of greens to browns (Manure, seaweed, cotton seed meal, Kelp meal, grass clippings, kitchen wastes (excluding any meat products), and so on for greens. Maple leaves, apple leaves, Linden tree leaves, newspaper, Birch and so on for Browns). Always finish with a green like manure then, you can plant directly ontop of that.
What I just described is called sheet composting this will slowly increase the quality of your soil year after year. I generally do this each fall and also collect extra piles of leaves (usually a pile of maple leaves and a mixed pile) for composting a mulching in the summer.
In the summer, spring and early fall you can also dig trenches and layer up the greens and browns again and then refill with soil. This is called Trench composting and aids in the speed of your soil improvment.
This seems like a lot of work but, I don't find to be. It's actually fun to be out collecting all of the free or really cheap soil amendments and then layer them up. It's really neat to come back to the garden in the spring and see this lovely new topsoil.
Good luck and be sure to read the other threads on soil improvment.
One note that I will make is that clay soil takes a bit longer than sand based soil to improve because the clay particles for soil aggregates with the added organic matter that are more recalcitrant than aggregates formed with sand particles.