As just noted, acceleration at 10% is the way to go. The best way to overcome the issue at hand is to have the lowest sensitivity. That way when you go from square back to x, when you slide your thumb, you won't let off the accelerate hardly at all.

With the airbrake sensitivity, initially when you set it up, try using 50-70%. I keep mine at 60%. The reason to initially decide on a setting, is so that you get used to the handling as you get better at the game. For me, changing even 10% completely messes up how I have learned to sideshift and airbrake. Some would say to lower it for zone, but I personally think you should experiment around in the 50-70% range and keep the settings set permanently.

I personally like to use the D-pad myself. There are many trains of thought over this issue, but I find it easier to barrel roll with the pad as opposed to the stick. I also personally prefer using L2/R2 for braking because you get a little more control over how much speed you lose when you brake. This is a small difference, but when you get up to Phantom, it becomes a huge difference. Even the slightest under brake can cause you to exit a turn too shallow, and the slightest over brake will often run you into the opposing wall. If you are using a craft like Icaras or Piranha, this technique is a must. If you plan on using an Assegai/Harimau/FEISAR, pressure sensitive braking isn't quite as important as just avoiding brakes altogether. While sideshifts are arguably easier using L1/R1, if you get used to using L2/R2, you'll find it easier to brake in the long run.

But still in all of this, you just need to find the right combination for you personally. The only thing that is a must is to set the acceleration sensitivity at 10%. There is no reason at all not to.