I agree, stirrup-less riding at all gates (including posting at the trot). I also like to practice the two point at all gait. I often do some sitting trot drills where I work without my hands. I don't know what your specific problems are, but I've some luck finding drills on YouTube to help with specific issues.
Out of the saddle, I look for gym work makes me a stronger rider - squats, triceps dips, planks, etc.