3 things: Practice, practice, practice!
And remember, even a triple time champion knocks down fences.
BUT -
Don't over-face your horse (jump too high of jumps for his ability), but never underestimate your horses abilities.
If you are new to show jumping, or your horse is new to it, or he/she is just not responding well to it, MAKE SURE YOU HORSE ACTUALLY LIKES JUMPING.
If he/she does, then ask yourself, "Is my horse bored, because I'm not jumping him quite to his cababilities?" or "Is my horse nervous/anxious/depressed about Show Jumping that I might be overwhelming him?".
If you answered yes to either of those questions, then decide - "Should my horse keep jumping, or have I ruined his desire to jump? "Should I jump him over higher jumps to keep his interest sharp"? "Should I start more slowly and moderately, and jump lower easier jumps, until he is ready to move up?" "Is my horse Show-Shy? Should I warm him up to shows gradually with an experienced show veteran until he is more comfortable?"
Also, is this horse the right match for you?
Are you a high level jumper/rider, and your horse needs a more beginner rider, or more training?
And PLEASE remember, Horses Aren't Push-Button Machines - they are beings with a mind of their own. Just like you and me.