Avoid these mistakes to build more strength and size and prevent injuries.
Don’t tap your feet or flail your legs as you bench. Instead, root your feet into the ground during the setup and build a firm foundation. Also, tense your quads and squeeze your glutes to stimulate total-body tightness and lift more weight.
Another common mistake is when people put their feet on the bench to “target their core.” The bench press is a strength exercise, not a core exercise — if you want to build a strong and massive upper-body, focus on lifting more weight with the bench press, not sculpting your abs. If you want to engage your core while sculpting your chest, try a Spiderman Pushup instead.
http://www.muscleandfitness.com/news-and-features/galleries/training/10-worst-bench-press-mistakes/slide/6
