Put 'em in a coffee can and add enough diesel to cover them and let them soak for a few days. They should clean up fairly well after that, if they're really bad they might need re-sharpened, but it's still cheaper than buying new ones.
I hardly ever use any clippers besides my 2 speed Andis anymore. I use several different models of blades; X-blocks are definitely the ticket for clipping heavy adhesives (legs, tailheads) but I still use mostly Opti-blocks or plain old blocking blades for most blocking, and I can't be without my 10 wides (they're great for clipping heads, and blending in necks). About the only things I use my old big Stewart sheep heads for is shearing with P2712's (or is it P7112's ?), or with a set of really sharp, beveled 20's when I'm building legs...