As I once read in an internet forum: "Yes, most professional bettors do. No, they don't want to share it with you."
People absolutely do beat the sportsbooks, and these people typically do have proven betting systems. However, as you can imagine, this is tremendously valuable information. If it predicted accurately on, say, a 60% clip, if you could bet tens of thousands of dollars on each prediction you would have an enormous expectation.
The reason I'm always leery of those "I'll sell you a proven betting system" sales pitches is that if it actually worked, they could make way more money than selling it to me for $99. Plus, if they sold enough copies to educated bettors, the information would be incorporated into the lines and become useless (look at "efficient market hypothesis" for details).
If you want to attempt it, there are steps you can take. First, read Sharp Sports Betting by Stanford Wong. It's a good primer on how to think as a "sharp", or educated/professional bettor. Afterwards, do your own research, come up with hypotheses, and test your model. Learn about data mining and data analysis to properly do this.
It sounds like a lot of work, and it is. And even with all the work, there's no guarantee you'll come up with a good betting system. But you're not going to see someone give one away for free on the internet and no one will sell it to you for two easy payments of $49.99. Unless you have a very good friend willing to let you in on his/her method, you'll have to develop it on your own.
As someone doing similar work right now (with some success thus far), I wish you the best of luck.
EDIT: To clarify, I'm talking only about sports betting. Bookmakers are not perfect, and I'm referring to finding opportunities where bookmakers put up inaccurate lines. For things like craps, slots, roulette, etc., there's no way to win long-term. Each bet is -EV, and there's no way to overcome that.