When I see people strength training at the gym, and working hard to lift a heavy weight or do those last few strenuous reps, it's hard not to notice their face scrunching up. If this sounds like you, the next time you lift, remember to relax the muscles that aren't involved in the exercise. It's the muscles in the face, neck, and shoulders that are most often engaged when they don't need to be. Tensing your jaw or shrugging your shoulders up to your ears won't make you any stronger. If you're working that hard that you can't help engage other parts of your body, then maybe the exercise you're doing is too difficult for your ability, or the weight you're lifting is too heavy. It may not seem like a big deal, but it all relates to having improper form, and that can lead to injury, which is a huge deal.
To make sure you're keeping your body relaxed except for the muscles you're targeting, ask a friend to watch you or look at yourself in a mirror.