Finding the perfect sound combination that has the lulls and crescendos and doesn't repeat too much is always a difficult task. I used to hunt for hours on iTunes trying to find that perfect sound clip, but would often pay for the rest of the song and be disappointed because it wouldn't fit my choreographic concept. Luckily, there are some great music download sites that don't require payment, only recognition of the artist.
I learned about www.incompetech.com during a computers in communication class. It is used by movie and advertising producers to complement their work. And it's all free, although donations are optional. So I thought, this would be a great resource for dancers, especially students.
Kevin MacLeod writes all the music on the site, unless otherwise noted. You can search various styles of music by genre or emotion, which is a wonderful tool considering many dances are conceived based on an emotion. The pieces vary in length from 30 seconds to 10 minutes and they can be downloaded via iTunes. If you can't find what you're looking for from Kevin's selection, try some of his friends' sites: http://www.jimmyg.us/ and http://www.jmtr.com/jmtr-royalty-free.html.