Know Yourself
If you don't know your limits, you can never grow beyond them.
The key to success is not knowing your strengths as much as it is knowing your weaknesses.
I thought this was fairly common knowledge, but some people I have worked with have yet to grasp the idea behind this. They are worried about disappointing those around them and claim they can handle more then they really can. In any work environment where you have to work with others, which is almost all of them, you have to be able to balance the strengths and weaknesses of each member of the group in order to maximum the chance of success.
When you know your individual strengths and weaknesses you can help move your group to a better place by not trying to do something that someone else in your group is better at. Or if no one knows how to do a certain thing, then you can look to expand the team to fill the holes. If you try to make up for holes in your group you run the risk of spreading yourself too thin.