Like the person above said, generally your stirrups are shorter for jumping, but apart from that, not a whole lot is different until you're actually going over the fence, when you'll take on a jumping position obviously. Also, for jumping courses, it can help to have a "light seat" - basically, sitting forward very slightly and lifting your weight out of the saddle a little to allow the horse to move more freely. This is used to a further degree for cross country when you usually sit forward more to encourage the horse to open out and cover the big distances more easily.
There are only two emotions that belong in the saddle; one is a sense of humour and the other is patience.
- John Lyons