See a dietician. Seriously.
Short of that, there's a lot of good ideas here in previous posts. Focusing heavy on protein is obvious... make sure you take in a good dose within 4 hours after working out. Also a good idea is to suck back a fruit juice or some other source of simple carbs within the same period, so your body can replenish your muscles' store of glycine used while working out.
There's a bunch of good supplements that'll help too. Creatine, chromium pecolinate, etc... again, dietician can be a big help there in indicating what supplements would be most advantageous to your body type.
Oh, and on behalf of all us endomorphs who have to bust ass and starve ourselves: go fuck yourself
