For our comps we usually pic a variety of music ahead of time and we actually play the playlist at the wall a few times during the semester or part of it to check several things:
1) Volume levels on songs, and figure out which ones need to be edited and readjusted so it's all the same level through the PA
2) Any questionable content that we didn't think or remember when we chose the song, so Nine inch Nail "Closer" is a nix.
3) Check for a flow and variety of styles... or as Kaleb said... a lil Sum something for everyone. As well as total time.
4)Finally we climb to it during setting, and If a song isn't working or doesn't get us pumped to climb then we cut it.
Several of us have PA systems and are former band members so we don't bother hiring a DJ as we play it right. Styles usually include Hip-hop, R&B, Gypsy-Balkan styled beats, House, Dub-step, some rock and roll, Indie and a bunch of misnomers... really whatever get people pumped is what works.