We're always being told what we should be doing a the gym: go faster, harder, slower; use a medicine ball; buy a band! But perhaps more important are the things we shouldn't do during a workout. In fact, when's the last time you paid close attention to your weight lifting form? Even the most savvy weight room warriors make mistakes, especially as they tire. So that you don't cheat yourself out of an optimal workout—or worse, bring your routine to a screaming halt—we asked experts to reveal the most commonly committed sins. Read on to see if you're guilty, and how to attain immediate…