As a general rule, you don't want to work the same part of your body two days in a row. When you lift you actually tear muscle fibers and your body rebuilds and reinforces them, which causes bulking and strengthening. You can use creatine or other supplements to speed the healing process, but I am unconvinced that it really helps. Steroids are the only sure fire way to speed up the process, but they also destroy your body at the same time.
Upper body one day, lower the next and don't neglect cardio...it will do you the most good in the long run.