It depends.Closed Betas ya don't have a chance in.Only people who live close to Sony HQ in the US can get into them.Open Betas you have to be invited.To be invited you might have to be active in the games community or something like that.
If you have a good enough PC you can volunteer for the Microsoft Beta Programme to test PC or 360 games. I've already gotten into some beta's myself
But remember, Beta's aren't what you may be thinking. They are very buggy, graphics aren't good and can be laggy.