Well, the first thing to consider is how does your horse trot? Some horses are just very bouncy and therefore hard to sit. The good thing about that is that the more you trot your horse, the better he will get at it. Eventually slowing down to a jog. But for now, just concentrate on sitting back in your seat, really make sure your bottom is as far back into the saddle seat as it can go. And dont squeeze with your thighs, squeeze with your knees. And make sure your feet are inline or just in front of your knees. Not behind your knees!! and heels down. If your stirrups are too short, you will bounce alot too. So make sure they are the right length. Stand up in the saddle and you should only have enough room to put 3 fingers between your butt and the saddle. Hope this helps.
If it's not half ass, it's just a horse!