well with the new plastic molding tech they have, there will be even less wastage in the future - however, untill then, here's a perfect use for your sprues:
http://uk.games-workshop.com/cityfight/rubble/2/
PS - I find it goes through the mincer better if the pieces still have bits sticking out of them, rather than cutting it down to straight bits of sprue. Also, a course grain gets the best results.