Something slow paced. Mozart has some pretty high-tempo pieces. Then again, a good march or fanfare might get you going and motivated :)
Usually something you know really really well so that you don't feel the need to pay too much attention to it and lose focus or something without words.
I would suggest some good ol' music from the Baroque period like Bach or something.

I suggest J.S. Bach, A. Scarlatti, G.F. Handel and Hadyn. Fugues especially, although pretty much anything will do. It's the complexity and structure that stimulates the brain, apparently. Mathmaticians study the music.

I believe that CD compilations of suitable classics are being made just for studying or creativity. Also new music is written for certain purposes, such as creativity stimulation, or sleep inducing. So you might want to try to find these sort of CDs.