Get yourself in good mental and physical shape. Work hard to build up your body, eating right, and get the proper rest.
Pay attention to what the coach says about your opponent and plan accordingly.
Know your opponent's style and his strong and weak points. Do not allow his past record or previous matches to destroy your confidence. Watch opponents wrestle other matches when possible. A match can be one or lost before you step on the match if you are not mentally prepared.
Specialize in at least one series of moves from each position (top, bottom, standing) and learn at least one other series. Have a desperation take-down move (preferably a throw) available if you need it.
Practice these moves hundreds of times to perfect them.
Know/learn the current wrestling rules for the style you are competing in.
Never sell yourself short or lose sight of your goals.
For general preparation, do more than what is required by the coaching staff, as well as working hard to do what is required correctly. Make a personal commitment to become the best that you are capable of becoming. Wrestle off-season if there is a program available. Go to all the open tournaments you can.