I'm not aware of any reason to hold triangle, that would show bunt and if playing online, might tip off the other team you're going to steal if the batter goes into a bunt animation.
As you said, you just hold L2 and release when the pitcher goes into his motion. I look at it as if I'm a runner on base trying to read the pitchers motion and release L2 at the right time. You have to have a good runner with a steal rating and time it right to get a good jump. Also helps if the pitch you go on is an offspeed pitch. If online, you can press L1 to get an extra step in your lead (only do this if you see your opponent just pitching quickly and paying no attention to the runner). If not online, pressing L1 will mostly draw a throw over and you might get picked off. It's not easy, but I guess it shouldn't be too easy or all players could have a decent shot at stealing a base.