Ah, I am a pro at that!
It DOES cause your legs to ache (trust me) the day after (mine still hurt, it's been a few days! LOL I did that for like an hour almost though), but it really helps with your leg position.
Keep your legs in the position they would be with stirrups (have 'imaginary' stirrups) & breathe, & lift your chest. It really helps. :) Also move your toes up & down as your horse moves; it improves your balance even more. Using your seat & feeling your horse move really does the trick.