OK - here's what HF-W says on the matter:
Feed them on a one item, purgative diet (sliced carrot, or lettuce, for example) for 5 days, then starve them for 2 days. Although he does say, if you're in a hurry, purge for 3 days then skip the starvation.
They can be prepared in one of 2 ways, depending on the desired end result. If you want them still attached to the shell, but out of it, place the live snails in unsalted, fresh, cold water, so they come out of their shells, then gently heat. Salt the water (once they're 'asleep') and bring to the boil, simmering for 10 minutes. The other way involves shaking the live snails in a washing up bowl or similar, so they retract in to their shells. You then plunge them in to boiling water (or veg stock, or whatever you fancy). Bring back up to the boil and simmer for 10. Strain them off, then once they're cool enough to handle, remove the meat from the shell with a pin.

Recipes:
Garlic snails - Fry some garlic in a shed load of butter, add half a glass of wine and allow to bubble loads. Chuck in the snails and bubble for 2 mins. Add chopped parsley and serve with french bread.
Snail gratin - Do as above, but without the wine, and use shell-less snails. Put snails in ramekins and top with breadcrumbs and grill (personally, if I were doing this one, I'd fry off the breadcrumbs in yet more butter first; makes it richer and more crispy.)
Snail rissotto - pretty much chuck garlic-prepared shell-less snails in whatever your favourite basic rissotto recipe is.

My own idea: use in a paella, once again, prepared in garlic butter. Would be a fine substitute for mussels I reckon.

Happy snail hunting!!