Practice Makes Perfect

One of the best ways to make sure you really know what you think you know is to practice it. I've learned long ago that the best way to prepare for a test is to do practice problems instead of just following the teacher's instructions without attempting things through your own thoughts. Thinking about something is good, it makes sure you know what you're talking about, it drills in what you know and lays bare what you don't know. You can reorganize your own opinions of your own ideas, and move on from there.
This works well for tests in school, but it's good for real life skills and events, too. If you don't practice what you know, you soon forget it. If you don't practice what you know, you don't know if you really know it. Practicing is a good way to hone your skills and discover your weaknesses before someone else does.
Practice might not make things perfect, but it will certainly help you get closer.