1. Maxing out every week Many figure out that doing their max a lot will increase their bench...WRONG!! This only breaks and tears down the muscles which causes your bench to "drop", which shouldn't have happened in the first place. Your bench isn't supposed to go down. To others, they do their maximum so much, it causes an injury. This shouldn't happen either. Your maximum is a one time deal or "once in a blue moon" thing. Once you do it once, you don't need to do it again, because if you do it hinders your attempts to increase your bench.
2. Doing nothing but chest all the time Doing your chest three times a week won't make it stronger as well. Your chest can only take so much work and it has to recover regardless if your training three times a week. Your need to work the other muscles that work the bench and beyond. This also causes your bench to go down, which again SHOULDN'T happen.
3. Having a "bench competition" amongst friends Now, I'm totally guilty for the next sin...having a bench competition amongst friends. In the gym, this happens. Somebody starts talking about benching more than their buddy and it starts a challenge. This is also a NO-NO. You don't need to do this and plus it brings your ego down even further. Even worse, someone can get injured. You want to challenge someone, do it at a bench meet. That's the perfect motivation for it. Don't try to prove something to your friends, if they don't believe you can't bench 405 at 18 years of age (I'm exaggerating), then they don't. Prove it to them when you hit 450!!
4. Benching directly to your chest The next sin causes the rotator cuff injury. When you bench directly to your upper chest, it puts a strain on the shoulders/rotator cuff. When you work up to doing heavier weights, you increase the chance of getting a rotator cuff injury. You need to bench below your lower chest. You get more explosion as well as strength that way.
5. Warming up incorrectly The next sin happens even to the best of us. When some of us warm up on the bench incorrectly. For example, some do too many reps with light weight causing them to fatigue early and it also causes them to not to achieve their maximal potential. This generates frustration and causes the lifter to give up. This is also why someone's bench goes down. Constant moving on the bench is also a sin. Its not a smooth motion and it can cause injury. Plus, if its causing you to do the "snake" motion, then its too much weight, and that's also another sin. People are not pacing themselves.
6. Using a “close grip” hand position The next sin is a new one for me. Most people teach this to the beginners which fools them. They start them out with a "close-grip" hand position on the bar, which is outside the spacing on the bar. This is a bench press, and one of the fingers has to touch that spacing. The close-grip bench press is good for only working triceps, it doesn't work the chest.
7. "Snaking" on the bench The final sin is what I call "snaking" on the bench. This is caused by doing too much weight and also caused by a huge ego. "Snaking" on the bench makes the movement more difficult and it could also cause injury. You don't want that. Drop the weight and the ego and go back to getting the perfect form.

Conclusion Committing one of all of these sins WILL halt your progress and stop you from achieving the biggest bench possible. There is no way around it. If you're guilty of some or all of these sins, you may need to drop the ego and the weight and start from scratch. Once you master the form, your ready to increase your bench.

Read more bench press articles at: http://www.criticalbench.com/training.htm