Originally Posted by sugarcube
I move forwards when he's ABOUT to jump
The clue to your problem may well be in that statement.
If you shift your weight (move forward) before the horse actually starts to take-off it will unbalance him (put on forehand) in front of the fence resulting in the horse having to put in a short (unplanned) last stride. This then puts the horse too deep to the fence requiring a much bigger effort to jump it.
The riders postion over a fence is simply to absorb the acceleration of the actiual jump and should only be adopted as the acceleration is occuring, ie the horse is actually jumping. It should also be a fluid movement from rider upright to the amount of fold required to absorb the acceleration and back to upright as the horse makes the arc over the fence.