I have my saddle checked and ref locked normally once or twice every year. but i just got a new saddle a few months ago to help better fit her. which has been doing an amazing job and it has changed her back A LOT. but just want to make her even more comfy.
Before you start more intense work, make sure your mare is warmed up and stretched out. Same with after your ride be sure you cool her out. Also massage her back as much as possible. Chiropractic work and acupuncture are good options as well:)
You could try lunging her with side riens or "bungee cords", this helps to encourage stretching through the neck and back too. :)
When you ride her try to let her stretch her topline as much as you can. This will stretch her stiff/sore muscles and make her feel better:) Also what the other people said
When was your saddle last fitted? It should be professionally fitted twice a year, in most cases.
Perhaps massage techniques? My horse had a similar issue and the massaging helped with him. Do you ride with a half pad?