I bet very occasionally at VIPSports.com.
The key is to find an area that the oddsmakers generally know nothing about, and become an expert in that area. Pro football is a waste of time, because most of the money wagered is on football and the oddsmakers make it a point to analyze every game to death. Pro basketball and BCS-conference football and basketball are about the same, although there are a few more opportunities. The prime betting opportunities come in the form of mid-major and minor conference football and basketball (i.e. basketball in what would be Division I-AA, and football and basketball in the MAC, Sun Belt, etc.), baseball and hockey. The first round of the NCAA tournament has some pretty bad lines as well.
There's a great book called The Odds that tells about one season in the lives of a professional sports bettor, a wannabe bettor, and a sports book manager at the Stardust casino in Vegas. A very good read for anyone wanting to know how these guys live.