Assuming you are a natty (natural bodybuilder) it would be pretty hard to gain slabs of mass and burn fat at the same time I think.Alot of people swear by the timed carb approach,heres the link
http://www.ironaddicts.com/forums/showthread.php?t=3398 ,others say full keto diets are the way to go.Either way I think it requres very careful manipulation of your carb intake and of course good solid programming too.I myself have very limited experience with the Timed carb approach, but will say that whilst doing it I still managed to gain lots of strength as the fat slowly melted away.I'd definately recommend trying it.Hope this helps.
Cheers HIThopper