Anything that works the core, back, and thighs- sit ups, planks, supermans, squats, etc. Yoga can also help a lot.
I've made up a blog post about good work out you can do to improve your riding position and strength with Meryl Wheeler, the creator of Pop Fitness. She holds a fitness program for equestrian.
Check it out, there are a lot of great things you can do.
Stirrupless or bareback for overall position and gymnastics/gridwork for especially two-point