Seriously, know your personal limits. I can't tell you how many times I've seen somebody give up too early or get hurt during training or racing because they simply had no idea what their real thresholds were. The whole idea behind training and/or competing is to push your thresholds to the limits to fulfill your potential. If you don't know what your limits are, how can you possibly know what your potential is?
One of the most misunderstood areas for new bodybuilders is nutrition. I talk to guys all the time that have no idea of their daily calorie intake, their daily protein intake, their carbohydrate intake. They have no idea of what types of foods they should be eating, or when they should be eating them. They don't know what supplements do what and what they should using. Let me refer readers to some of my other articles that detail these areas.