First off, I'd say maybe the best place for this question is somewhere like Wetcanvas; I think Larry Seiler is a moderator there and he did lots of good outdoor scenes.
Aside from that, the best way might be to start with an analysis from the large scale on down to tedium. In the conditions you describe, and depending on the sand - the sand in sunlight would be tending to whitish (say a light transparent ochre over white) while shadows would ten to be bluish over the natural colour of the sand, since they are being lit by the sky but not the sun - but still bright.
Then you have to look at how the shadows are formed - sand on can't sustain much of a slope unless it is compressed or wet (in which case it will be dark), while on the other hand it is formed into ridges and ripples by the wind (often in semicoherent patterns), or lots of little indentations by feet. And birds. When the tide is receding, it will be smooth for quite awhile, and dark until dry.
As for the granularity, you might try dry-brushing in areas lightly with your shadow colour over your lit colour; the small shadow are just tiny flecks - and ditto for some highlit areas..
I guess the best way is to take some scratch pads and head someplace nice, like the Riviera. Better yet if you take along a (paid) consultant - and I'd be willing to sacrifice a bit of time to an artistic cause
Cheers;
Chris
PS. I'm doing this off the top of my head, so you are still better off asking at WC...
