Incorporate as much organic matter as you can lay your hands on.
I second that. The more the better.
Has anyone tried that simple DIY soil particle test? I find it very helpful when modifying the soil. Just fill a jam jar with soil, top up with water, give it a really good shake, then let it settle. after a while you will see different layers, from which you can tell the proportion of clay (the tiniest particles that settle on top) to sand (at the bottom). You can also see organic stuff floating on the surface, and mixed with the large particles.
I started out with a lot of clay and virtually no organic material at all, but that has changed! I now have mostly organic material, and very fine it is for growing in!
Of course, this sort of test tells us nothing about nutrients, but I'd say that with an abundance of organic material you can't go far wrong.
Organic material not only adds vital nutrients and helps drainage, it also delays drying out in hot weather. Sand really does help drainage in heavy clay, but I feel it dries out too quickly, and doesn't provide nutrition.
I'd go for organic material any day.
