Well, lower leg is pretty hard to get still. But when you do get there, I must admit, it helps you heaps in all levels - flat classes, jumping classes ... you just overall get a much more neat and good picture. It is a sought after goal, though, though I do have some suggestions.
Heels down, toes up, really just comes with practice. Practice, practice, practice. Practice without your horse, even - sitting on the couch, playing on HF - keep your heels down. Working in the office - keep your heels down.
Standing in line - see if you can stand on your heels. Just practice, and you'll get there :) You won't even have to think about it, eventually.
Okay - as for lower leg. First off, you need to get your horse used to you gripping with your lower leg, (NOT knee! More on that later!) because that will not only make him more broke, but be your only way to really still that leg. So, during rides, 'hug' him with your leg. Don't have any intention to speed him up. Just hold your leg against his side. If he gets mad, and tries to speed up *at all* stop and back him. Stop, back three steps, then make him stand for 10 seconds. Never take your leg off. Then cue him to move off. There should be a diffirence. If you don't feel the diffirence in holding and squeezing your leg, you are holding too tight.
One that is taken care of, make sure you are moving with your horse well. The fact that you were stiff before might make this more of a challenge for you, but it will feel so much better once you get the hang of it. Rock your hips with the motion of your pony, just like you were on a rocking horse. When walking, try feeling which leg he reaches forward with without looking down. Just think to yourself, "left, right, left, right." This will require you to move with your hips, or you won't feel it.
If you are moving fluently with your horse, and holding with your legs, it's time to up the antee. Working out. Go to the gym, and do all the leg excersizes, taking breaks in between, doing something to work your core/abs. Both of these things will REALLY help your posture! For example, do the leg lifts, then do sit ups. Then do a different leg excersize, then a diff. core excersize. This is called super setting, and is the fastest way to build muscle.
When you can't make it to the gym, excersize at home! Every gym-less night, do 100 sit-ups and 100 leg-lifts. You will be well on your way to perfection!
Remember, horseback riding is a sport. It takes conditioning, and it takes work. I'm sure you've figured that out by now ;)
I've been riding for 8 years, but only started working out within the past 3. I already had all the other bases (heels down, relaxed in the saddle, holding with my legs) but still had a little twitchy leg. After 3 years of working out, I came to this result - and it's still not perfect -
EDIT: Holy cow, that ended up long!! Hope you got some good stuff outta there xD Ignore the intro of the video, with his name and such. I posted the same video for critique a while back. Anything you see now is much appreciated, as always :)