The moment your arms start swinging like the storm door, Houston, we've got a problem. Keep them folded, or if they're holding dumbbells (well done), keep them straight. Also, like the broken record we are, please keep your back straight. It likes being straight. You look better with it straight. Friends don't let friends bend backs.
This is so difficult it almost begs the question, how can there be any cheating if most can't even physically do it? Fair. But there are some no-no's, like, should you be able to get your entire body of the bench, please watch your neck at the top, and your back on the way-down. This means not going past perpendicular with the bench when up top, and don't curl your back into a semi-circle on the way down. You avoid doing these things, and you're still able to do the exercise? You either an Olympic gymnast, or our new hero.