Correct position is correct position, no matter what saddle you're in. At slow speeds, you should be able to draw an imaginary vertical line through the rider's ear, shoulder, hip, and heel. At higher speeds, you need to close the body's angles, but the rider's center of gravity (midway between belly button & crotch, and in the middle front to back) must remain over the feet to be in balance.

When people have problems going from western to English, it's usually because they do not have proper form and balance in the western saddle. I have long said that it is equally difficult to ride well English as western. But it is much easier to ride badly in a western saddle, as there's more to lean on and hold onto.

There are as many ways to teach correct balance as there are riding instructors and students. A visual learner will do best watching someone else ride correctly, and watching her own efforts in a mirror or on video. An auditory learner will appreciate detailed explantation. A kinesthetic learner needs to experience balance--standing on a mini-trampoline with her knees and hips slightly flexed gives her a chance to move in and out of balance--just be sure she has something solid to grab onto when she gets too far out of balance. On the horse, she'll benefit from having you put her legs in the correct position--and move them back there periodically, to refresh her sense of that the goal is.

Posting to trot is the art of moving one's body in time with the horse's stride. Although we commonly talk about it as up-and-down, correct posting is also a front-to-back movement, and has a bit of side to side as well though that should not be emphasized. For a rider who is already familiar with sitting to the trot, one way of approaching posting is to have them "let the horse toss you out of the saddle for a single step". Sit a little bit, then repeat. Gradually decrease the number of sit steps between the ups, and after a while, they'll be posting. Good idea to have the rider hold on to a grab strap, neck loop, or a handfull of mane while doing this, so they don't hit the poor horse in the mouth when they loose their balance (that's when, not if--they WILL flounder around up there). Easy grab strap is a western curb strap buckled to the two loops at the front of an English saddle.

Leg muscle strength for posting and two-point takes a long time to develop. It comes from many hours in the (English) saddle, or bareback, practicing posting and two-point. Gym exercises like squats, and core-strength training like yoga and Pilates are good additions to the program. The ideal way to build both strength and position is to ride on the longe line, IF you have a horse who is trained for that, and a longeur who knows what they're doing. If the local definition of longing is chasing the horse around on the end of a rope, then scratch this idea. The advantage of properly done longe work is that the rider can focus on herself, and can hold the mane or saddle for balance if needed without worry of pulling the reins or having the horse run off. A good longeur can keep the horse at a very steady pace, which makes learning to post much easier than if the horse is erratic.