Do some bareback riding if you can, you get a much better feel for the horses movements and heaps of stirrupless work. Most importantly RELAX, you won't go very far if you tense so just relax into the movement :)
try to let your legs just hang if you are doing stirrup less and really relax in your hips and seat as this is an important shock absorber aside from your knees.
Take stirrups away. Do off horse exercises to build core (yoga, pilates etc) strength. I used to practice in the bush holding onto the pommel, let the horse just trot along in a straight line in soft sand and get a feel for it. Start slowly and build up.