Just gotta practice... If you are backlashing, and the brake is set decently you are probably just trying to cast the bait to far. I know anytime I try to really musle a cast, 2 out of 10 times I'll get a back lash.. I only thumb the spool when the bait is about to hit the water (If I want it to land a bit shorter) but when you are just learning, you can thumb the spool the entire time. You wont be launching baits this way, but you'll get the hang of it. I have my right hand on the reel (obviously) and my left down near the butt of the rod. Gives a good pendulum motion when making a cast, kinda like a golf swing.
It's like riding a bike.. One time you'll just 'get it' and you won't even think about it anymore.