trotting!

From your heels up:

Your heels should be down, absorbing all shock.
Your legs should be stretched, keeping gentle contact but not gripping.
Your hips should be moving loosely side to side as the horse moves its stride.
Your back should be like a slinky, up and down,also loose.
Your shoulders should be back.
Your neck should be long.
Your chin should be up.
Your face should be smiling.

I'm learning in the trot, it's so important to let your pelvis move with side to side motion of the horses legs. So that pelvic triangle is moving not just front back but with the horses legs, left right as well. I try to use my ab muscles to keep my shoulders and back straight while allowing my bottom half to become a part of the movement of my horse. If you consciously have to make an effort to keep your heels just so, perhaps your stirrups are a little low. Even though I'm long legged, I like my stirrups a bit high right now.

When getting riding lessons, the instructor told me to post... on a Western saddle.... .-. I looked so awkward. Lol.

Duck tape yourself to the saddle! Haha no actually what I did when my horse would absolutely not slow down for me, I would make her go really really fast for a while! When she wanted to slow down, I would speed her back up! This method probably doesn't work for horses that could run forever though. If your horse is going the correct speed for wp, you should have no problem at all for sitting though. Is your horse paying attention to you? Are you sure you have the right bit for her as well?

Update!! I am now a sitting trot pro!! Hehe :) I have been working on it for a while and now I've got it!! Thaks for all the advice!