I admit it. I love to watch myself when using weights. It's not because I love staring at my toned muscles; it's because I'm checking my form.
It's great to make sure you're doing strength-training moves correctly, but if you turn your head to take a peek, you could strain your neck or upper back. So instead of glaring over at your deltoid, stand in front of a mirror to watch yourself, making sure to keep your neck in line with your spine. You can also have a session with a fitness trainer so he or she can watch you do your moves and give you pointers on how to perfect your form.