When you put your heels down, try to point your toes in a litttle. Just a little. Not pigeon toed. Also squeeze with your calves so that there's little air showing between the saddle and your knees. Your shoulders and heels should be in a straight line. To strengthen your legs up to make it easier to grip the horse, try riding without stirrups about once a week. Make sure you're posting the trot to really strengthen up those muscles. Pretend you have stirrups when you really don't-- keep your heals down, and don't let your leg slack. Hope that helped! I've been riding english for over 9 years.