If the jumps are little or your horse doesn't need to make an effort continue posting . If it's bigger go into your two point . Look straight ahead.
Think of it more as moving up and forward. Think of a string, attached to your pubic bone, or if you prefer, to just below your belly button. that string goes off to the distance, toward where you are riding, and it pulls your pelvis up and forward :) Be careful of your stirrup's length as well!
The best way to learn to post correctly is to go out on a trail rode and trot for most of a two hour ride!