For one, check your saddle fit. Unless the swayback is severe or the horse is otherwise uncomfortable they can be ridden like any other horse, just make sure your saddle fits the horses back. However, I'd usually deter any inexperienced riding a sway backed horse if possible as it is more detrimental for them to be ridden hollow than a standard horse.
Next, focus your workouts on the abdominal muscles when possible. Out of saddle you can do belly lifts and carrot stretches, under saddle you can do leg yields, leg yield circles, and hills.
"The trouble with the world is that the stupid are certain and the intelligent are full of doubt"