I trot while having only one foot in the stirrup.. and alternate. In the beginning it was a nightmare but it really helps your balance in the saddle.
Try using a rod or pole in between you back and arms to keep you sitting up right with good posture. My trainer always made me do this every lesson and it helped me so much!
Do double posting...basically instead of posting every beat (up down up down...) post 2 up then 1 down (up up down up up down...it will improve your leg strength and position and once it gets too easy take the stirrups away.