Q: What are the rules for working out when you're still really sore from the day before?
You should NOT train a muscle that's sore. The reason for this is that your body interprets exercise as stress; it's during the recovery time when the real work is being done. Think of exercise as the architect and recovery as the builder. If you work a muscle too soon after a training session, therefore, you will just break it down.
Also, you shouldn't be sore for more than two days after a workout; if you are, you may have a slight injury or muscle strain. If this is the case, take it easy and give that muscle time to heal before training it again.