In some ways yes. Firstly, your stirrups will be shorter. You still want your heels to be under your hips, but the angle of your pelvis is different when you jump. Jumping saddles are built for your pelvis to be tilted forward, so more weight is on your pubis than your seat bones [they should hold equal weight when riding dressage] this is also why you ride the front of a jumping saddle more.
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.
I'm with sapphire here. In showjumping you are basically a dressage position in a light seat slightly tilted forward. Whereas, cross country, you would use that position about 5 strides before the jump but most of the rest of the time your out of the saddle in 2point position. So butt out tilting forward. It gives your horse move freedom to move and cover more ground. Cross country is timed so you need to cover ground. 3kms in 6 1/2 mins is fast.