I don't have any good tips about heels down, but I do have a question about posting in general. Why do we post at the trot? I can sit extremely well on most horses at the collected or extended trot. No bouncing, etc. Is posting for the benefit of novice rides to keep them from bouncing the heck out of the horse's back, or is there some other reason?

I think it depends on the horse and the rider and the discipline. Some horses have a nice smooth trot, if you watch their top line while they are trotting.... it’s smooth and the up and down motion is not bad, others well kidney busting is a good term. And if the rider is relaxed, absorbing the trot is easier, and when you relax posting comes easier as well. (Relaxing is not the easiest thing to do when trainers are yelling “heels down”) I think lower leg relaxed is more important than heels down.(personal opinion) I'm not saying point your toes down but I wouldn't get carried away with forcing your heels down, I see people do the heels down thing to a point where their legs are out of position for applying any aids. (Dogma is never good)
If you’ve ever been to a Saddle bred show some of the posting looks like it would be exhausting. But then it is for show and a hell of a lot of fun to watch. Then watch Isabelle Werth sit the trot and she looks like part of the horse.
Personally I like to post on trail rides as my beast has a wonderful ground-eating trot (about 10-12 mph) that is just a bit uncomfortable to sit, especially after 10-15 miles. My grand father was cavalry and his trick was practice with no stirrups with a handkerchief between your knees and the horse, if your toes went down you lost the handkerchief as your contact point lowers to your calf.

Hi Amy,
Here are some tips I've used on students in the past with success.

Close your hip angle slightly so that your weight is just slightly forward and with the horses motion.
Instead of trying to shove your heels down, make sure that your hips are over your heels and try pulling your toes up. Relax your ankle as much as you can. Glance down at your toes. You should just be able to see the tips of your toes past your knee.

Then, when you are posting the trot, try not to rise too far out of the saddle. You want your lower leg to stay steady and you just want to rise slightly out of the saddle and forward. This should come mostly from your thigh, but take care not to grip with your knees.

Some exercises for getting your heels down include:
Standing on a step at home with your toes on the step and your heels hanging down to lengthen and stretch.
Two point does work wonders if done properly. You can do this at a walk and a trot while you are warming up. See if you can keep your hips over your heels and balance in your two point that way. Make sure to keep your back flat, shoulders back, and your hip angle slightly closed. If your horse is well trained, see if you can balance in your two point that way, bridge your reins and while balancing, reach with the other hand and touch your horse between his ears at his poll, then try again touching his croup (top of his butt, kinda hard to explain).

Hope that helps some and have fun riding!