Mistake 1: You use the wrong weight

The goal is to challenge your muscles, not to simply go through the motions. If you are able to complete 15 repetitions easily, then the weight is too light. On the flip side, if you aren't able to perform an exercise through its full range of motion, and find yourself cheating on form, then the weight is too heavy.

The correct weights will feel challenging by your last few repetitions, but won't force you to sacrifice form.

Mistake 2: You do the same...