You can learn to stop arching your back while bench pressing. I'm a certified personal trainer. The first thing you can do that will make it easier to stop arching your back while performing the bench press, is to watch someone else committing this mistake.

Yes, watch them, and you will see how ridiculous it looks. That's how you look. Sorry for being blunt, but when a person arches his (or her) spinal column while bench pressing, how can he not know that this fools nobody, and isn't a true bench press?

Sometimes, the back arch is done during the bench press because the person wants to be seen handling a heavy barbell, and the only way to move the barbell is to arch the back. Even if the barbell is heavy for your body weight, you won't impress anybody.

The next step is to abandon the idea of handling the weight load that you arch your back for. If you can't maintain proper form with a particular weight load, then stop using that weight load. Bad form with heavy weights won't progress you as quickly as good form with lighter weights.

There's a woman I see at the gym who dramatically arches her back when bench pressing -- the barbell is 135 pounds. She wants to be seen bench pressing 135 pounds. However, what everyone sees is an exaggerated vertebral arch, and not only that, but she brings the bar down only halfway for every rep.

Do you do this? And if so, is it to be seen handling a heavy barbell, or do you really believe that a big back arch (especially coupled with incomplete reps) will make you stronger and bigger? It certainly doesn't make you look strong, and it will do very little to advance your strength or grow your muscles.

Use lighter weights so that you are not tempted to arch your back when bench pressing. People don't care how much weight you have on the barbell as much as you think they do.

The next tip is to place both feet on the bench, legs bent, while lifting. You won't be able to arch your spinal column this way. Keep your feet on the bench for the entire set. The flat back will force your chest muscles to do much more work than if you arch your spine while your feet are on the floor.

Arching your back while bench pressing is called a muscle substitution pattern, and that's exactly what it sounds like it is: Some chest muscle fiber gets substituted out by leg and lower back muscles when the bench press is performed with an arched back. When you bench press, do you want to build your chest/arms, or lower back and legs?

references:
weight bench
Best Weight Bench