If your balance is questionable, sit tall to the base, count your strides (3,2,1) or rythm (1,2,1,2) I keep light contact with the reins. At the base I usually add leg. If you are more experienced, you can lighten your seat by shifting to a half-seat, but if there's a possibility you can throw your horse off balance, don't. The half-seat or a light seat is needed to help your horse over taller fences. In the air, make sure your weight is in your heels, your legs are wrapped around your horse and your knees aren't pinching (this can prevent leg swinging) and make sure you release. Depending on the height of the jump and your ability and balance, you can do either a crest release or an automatic one. If you are a beginner or a novice and are worried about catching the horse in the mouth PLEASE grab mane at the base or during lift off. That way, your hands wont jerk on their mouth. As the horse lands, take your time coming down, try your best not to slam the horse's back or land too early because that can cause the horse to knock a rail or cause him discomfort. If you were holding mane, you can keep hold until the landing stride. After the jump, maintain balance and rythm, and ride on towards the next fence.
A few strides before the fence, you can look at the jump, but before 3 strides out, look up ahead or at the next jump. DON'T LOOK DOWN. Bad bad habit. Especially with a horse that hesitates or refuses. Looking down can make you drop your seat, round your shoulders and communicates to the horse "stop". This can either create a refuser/hesitater/chipper or bring out already formed issues.
Counting can really help you focus on the task ahead and not get nervous, ahead of the horse, or left behind.
Once you're 3 strides in, try not to mess with the horse. Try your best to adjust the strides and fix your approach before then, so you're not scrambling to the base of the jump.
BREATHE. I have a bad habit of holding my breath when I jump and it really helps when I think about breathing.
Can't think of anything else.. I feel like I'm missing something though xD
Hope it helps