The posting seems forced because your center of gravity isn't over your stirrups. There are several options.
1 - You could bring your heels back toward your hip. To do that, your thighs need to be more vertical. One suggestion I liked was after sitting in the saddle, grasp the front of the saddle and try to shove your knees down low. You may or may not need to lengthen the stirrups to do that.
2 - You can lean more forward. This is a picture from a George Morris book (although at a canter):
3 - You could also post at a diagonal. By that - and I doubt this is proper English riding - I mean you can stretch your legs some and get your weight off the saddle, but leave the line from your heels to rump as a diagonal, instead of vertical. That works OK for some situations, but it masks the problem: your center of gravity is too far behind the stirrups.
Cantering can be done in a half-seat. That is how I start canters, and then I can try shifting my weight toward the rear once I get the rhythm down. I got a lot out of this thread from a year ago: Riding the canter in half seat
For honesty's sake, I'm the chubby guy riding the Arabian. My Aussie-style saddle rides almost identically to my Bates Caprilli AP saddle. I go with longer stirrups because my mare tends to do the OMG Crouch and drop out from under me...
She was doing her 'Lazy Girl Jog' in this photo:
Good luck! I'm mostly a western rider with a very *******ized approach to riding, so take this post with a truckload of salt. My favorite book on riding is http://www.amazon.com/Common-sense-horsemanship-distinct-schooling/dp/0668026022/ref=sr_1_2?ie=UTF8&qid=1365084389&sr=8-2&keywords=Commonsense+Horsemanship.