Relax and imagine that you are heavy in the saddle. Get your heels down low but do not tense your legs! Make sure you're not hovering in the front of the saddle (I used to do this too and sometimes still do) and that you're pulling your shoulders back!
Open up your hips so you can roll with your horse and absorb the bounce with your ankle by keeping your heels deep down in your stirrup. Another key is to not tense up your legs or close up your hips because when you do you're going to bounce EVERYWHERE and that is very unpleasant.