something i forgot to mention before: if you are new or still relatively inexperienced with lifting, one of the best things you can do is a quick study of skeletal muscle anatomy. I found it very helpful to be able to visualize muscle contraction throughout an entire controlled rep every time, especically if its one where you can't watch yourself in a mirror. It also helps you focus more on areas where you may have an imbalance. One of the best books I've seen that incorporates this is Strength Training Anatomy, which you can buy here: http://www.amazon.com/Strength-Training ... F8&s=books
. check out the 'search inside' feature. it also covers safety and injuries in addition to technique and muscle use.
make sure you get the second edition if you buy it.