lots of topline building work. have the horse round up and push forward from behind, rather then pulling from the front end. i dont know what type of riding your planning on using her for but lots of dressagy type exercises would help. also, try alot of "long and low" like, have her stretch her neck long and low to the ground at the walk trot and canter. stretching is very important especially with an older horse. you need her back muscles strengthed, however because of her age, getting a swayed back is bound to happen.
alot of times you can fix it with topline excerise and muscle building, but sometimes its just due to age or genetics.
on an off note, i always recommend to people to put older horses on a joint supplment with HA, it helps keep them confortable longer