Quoted from craif:
I've developed a serious appreciation for machines with obnoxiously high tilt settings that really force the issue of relying on precision over percussion.
^^^ This, more than anything else, will make you a better player. I used to have my tilts 'homeowner style' (i.e. really loose or not installed at all) and thought I was King Shit at playing.... someone hosted a tournament at my house and warned me to tighten them up, which I took to mean "install the tilt bob". (haha)
The ball times were seriously long for those better players and it helped me not at all. That was my first tournament playing experience. Since then all the tilts are ridiculously tight, and you learn to play within those parameters and get better a lot faster. I won't say I rarely tilt now, but most of them end up being rage tilts.
Also learn the games and do not shoot things that will tend to vector towards the drains, and learn ball control with the flippers.
Slapping the side of the machine is mostly "safe" because it's not going to move the tilt bob much if at all and will still affect the ball the way you want - but you have to have the timing of that move PERFECT. I don't think that's the skill to focus on to get better... flipper ball control would be the highest skill there, and then of course aiming.... if you can stop the ball on the flipper but can't make the shot, eh.
Also, don't lean on your hands if you can avoid if to keep the wrist pain down.