Forgot exactly where I picked up on this, but it proved to be a very important part of my pre-game routine at Georgia. This really helped me as a young quarterback, with my inner confidence. And most importantly, anticipate the quick decisions that need to be made as a quarterback. Basically, this technique "puts you in the moment", before the game kicks-off.
  Positive visualization. I give it my full endorsement.
  Prior to the games, when the rough and tough linemen were banging on the lockers, I retreated to a quiet place in the locker room, put a towel over my head, closed my eyes and started visually going through the plays, one at a time. I'd run it through several times, against different defenses (Zone, Man, Blitz), and imagine myself making the correct adjustments. 
  For example, on the bootleg play...I'd visualize no backside pressure, which would allow me to get outside and look for the TE crossing, or a deep comeback by the WR. Then, I'd visualize quick pressure, and visualize getting rid of it quick to the flat route. Then, I'd imagine it was a big 3rd down play, say 3 and 5...and I'd see images of me running for the first down. 
  It was always positive. I would always see myself making the correct decision and making a good throw. It would take about :15 minutes. And I would always feel more confident. Like I was ready to go. 
  For a QB, on the field, plays unfold so quickly. There is no time to think your way through it. The speed of the game is way too fast for that. A good QB anticipates and reacts accordingly.