I've been there too! The good news is that when you learn to sit his trot, then you'll learn to sit most trots!
First, having your horse work properly over his back makes a big difference. Before you attempt sitting, make sure that he is supple over that back.
Second, I found that working out at the gym, particularly core work helped immensely. I was able to "hold myself into the saddle better". Bouncy horses hold a challenge because you do need to keep relaxed with the movement, BUT you also need to stay toned to prevent yourself from moving too much out of the saddle.
Another thing is keeping a good position, which seems obvious, but it is so easy to move out of position if your horse is bouncy. Most often, it is easy for the legs to slip forward, which then causes you to slide back and off your seat bones. Whenever you feel your leg shift out, then I like to think of moving my upper thigh down and back. Another thing is upper thigh position, which will often try and rotate outward, but you want your inner thigh to be in contact with the saddle. Then, your joints need to act as shock absorbers. You need to go with the movement.
Lastly, lots and lots of practice. At first, just do so in small spurts (a few strides of trot at a time). When you and your horse both feel comfortable, then ride a few more strides. You need to give yourself time to adjust to the movement.
And a few accessories that could help: full seat breeches and Sekur-Grip.