A common problem is clenching the jaw. It is a bit hard to spot and a lot of pianists underestimate the detrimental effects this kind of tension has on playing. An easy way to spot it is by practicing with your tongue out, between your teeth. Without a doubt, you will notice when you bite down on your own tongue. A good way to solve it is by chewing gum while you practice and paying attention to how your shoulders, neck and face feel when you play.