Because the bench press is the most popular weight training exercise, many lifters want to increase their bench strength.This causes many impatient weight trainers to do anything to get up more weight when they bench. Unfortunately, this leads to many lifters not using

proper bench press form.
If you work out at a gym, you've almost certainly seen lifters that don't use proper bench press form. They're the guys who have their entire bodies, besides the upper back, raised above the bench to decrease the distance they have to lift the weight when they bench press. even worse, they may be pretty much dropping the barbell on their chest to get it to "bounce" off of them so they don't have to deal with that sticking point on the bench press when you start the weight upwards. Besides the fact that they aren't really doing anything for their chest strength and size, they are also risking injury. To build actual strength and size in the pecs safely, you must use proper bench pressing form.
One thing about proper bench press form is that it's not a bad thing to decrease the distance you have to lift the weight a little bit. You are doing yourself a disservice if you try bench pressing with a perfectly flat back. You can arch your lower back a bit, but you have to keep your butt on the bench at all times. You should also flex your upper back when you're bench pressing. Contracting your upper back muscles wneh you're benching will pop you pectorals out so that you are actually working the chest muscles better.
For proper bench press form, you need to lower the barbell with complete control of the weight. Not only is it dangerous to practically drop a heavy weight on your chest, it's also inefficient for building chest size and strength. You need to take advantage of the negative part of a bench press rep.
Once you lower the weight to your chest, concentrate on exploing the weight upward for proper bench press form. This will help start a heavy weight up. Depending on how heavy you are lifting you should either lock out at the top of a rep or come very close to locking out.
When you're performing a bench press set, keep doing reps until you reach failure. Make sure to have a spotter watching you for safety, and to help get an assisted rep or 2 every now and then.
Once you get proper banch press form down, you'll be on your way to a stronger bench press and thicker pecs.
