Because this question comes up so much on this forum wanted to pass along a great music licensing resource that's right here in Atlanta. Melissa Love runs a company called Song Finder. Basically she handles all the licensing of music for all manner of productions from feature films to weddings and corporate uses.
Basically you just give her the wish list of songs, the use and a few more details and she tracks down all the necessary parties and negotiates the fees. We're going to be working with her both on our broadcast work and a new venture we're kicking off.