THE ARCH- A good back arch is essential to many lifters. The key is to move your butt as close to your shoulders as possible without bringing either off of the bench (feet planted). You want most of the arch to come in your upper back not your lower back. This will do three things for you. First, it will transfer the weight from your delts to the lower pecs. This reduces the risk of injuring your shoulders. NOTE: If you are putting too much strain on your lower back, don't do it. You will also notice that if you have a good arch you will be bringing the bar down just below your pecs. This makes the bench more like a decline bench where most people have more power. Finally, you'll be reducing the distance that the bar will travel during your bench press.
THE SHOULDER TUCK- Many powerlifters do not realize how much energy is wasted by allowing the bar to travel further than they have to. Next time you get on the bench at the gym, notice how far the bar is traveling. Then get yourself set, pull your shoulders back as far as you can, and take the weight off the rack. Try to pull your shoulders back a little bit further and keep them back as you lower and press the weight. With this method, I have taken as much as 4 inches off of my bench while creating a solid base below my shoulders.
GRIP- I am a firm believer in trying new grips from time to time. The grip can change so many factors in your bench press. You do not want to change your grip just to close the gap, but sometimes a slightly wider grip is just what the doctor ordered to cheat an extra few pounds out of your bench. Do not assume that just because you lose power when your grip gets TOO wide that you can't try it in a bench shirt. If you find that you gain power toward the top of the lift with a wider grip, try it with your bench shirt on. The shirt will become tighter at the bottom due to the excess stretch from the wide grip. This will compensate for the weakness at the bottom. Then you will gain power as the bar travels upward. Finally, you have closed the gap a couple more inches.