I agree with @maximumimpact
. It helps a lot. Jeremy steinberg got me to do that in a clinic to keep my legs aligned with my hips and it really helped me.
if you stand in your stirrups occasionally and find your own balance, that should be where your lower leg sits in a good saddle :) Just have a friend tell you when it slips forward and dont be too hard on yourself, habits are hard to break, it will come with time and strength
Riding without stirrups, lots of light seat. My instructor also makes me take only one foot out the stirrup and then trot a few circles and serpentines and then switch feet, do this on both reins so each foot has a chance to be inside and outside. Off the horse you can also do lots of core and leg exercises - Pilates and Yoga is great for riders
Lots of no stirrup work (if youre comfortable really) and as always listen to a trainer