Stretch their front legs forward, this relieves pressure from the girth and helps prevent girth sores/rubs. While riding bending your horse to the outside and inside. Also bending their head around before and after a ride!
Carrot stetreches for their neck and topline, I never get on before stretching front legs, stand facing your horse and pick up the front leg, I typically hold the knee and pull the leg foward so their elbow is at a 90 degree angle, can also switch and have the leg straight with elbow at about 45 degrees and pull the toe up but remember to support the knee as well. My guy gets tight overy his croup as well so I stretch his hind legs foward and back. To go foward I hold his ankle and stretch him towards the heel of his front foot and to go back I hook my arm under his hock and encourage his leg to go straight back, be sure not to push/pull to the side.
Walking around on a free rein and letting them stretch their head down as low as they want in the walk/trot will help to loosen them up and get them tracking