For me - epic musics.... none of this dubstep dirt...
But other things too:
->Keep it simple!!! You don't have to mod the crap out of it - just do a few clean, well exectued ones.. and just make a very good looking car - this can be great motivation... even if you chop an already modded base...
->Eat well, and drink well... they say you need to do that for exams.. but seriously - helps with chopping too
->Don't force yourself... if you don't feel like it.. just don't do it. Sit back and relax... do something different for a change then come back to it