Thank you all SO much for your advice -- I wasn't expecting so many replies so soon!
I will have to print this page and keep all your tips with me so I can try them out and not have to worry about "did I forget one..?"
So here's my feedback...
Some of my fellow Wranglers will take their feet out and rotate their ankles on the trail, too, but I'm not sure if I'm comfortable doing that myself. We have horse-eating deer and killer ducks on our trails, and you never know when something will jump out and scare the crap out of your horse.. Lol. I like to have my feet in all the time, just in case.
I think if I were to lengthen my stirrups even one notch, trotting might become scary. There are only a couple horses who I can sit their trots the entire time, and the nature of the business is that I rarely get my pick of who to ride. Customers first, then the Wranglers take the leftovers because we're (supposedly) more experienced and (should be) able to ride any horse. A lot of the time customers get put on my favorite horse, since he's so easy. :(
As far as getting my own tack goes, I would LOVE TO, but I can't until I have my own horse that no one else rides. We tack up at about 8 in the morning and each horse has his own particular saddle that he wears throughout the day -- no switching around, unless we have similar-sized horses that can trade. Nobody has special stirrups, unfortunately, and they're all different.
Talking about proper posture and leg position, my lesson instructor never mentioned anything except "from the knee down, no part of your leg touches the horse. See?" He was a pretty bad teacher, and got impatient a lot of the time so I just stopped seeing him after I'd learned a basic seat and how to keep my horse between me and the ground. The rest of my riding skills (intermediate, admittedly...) have come from working on the ranch and watching my coworkers. I never actually had the one-on-one "lesson" environment for the very fine details... I wish I did.
My boots are the basic-model Justin ropers... which I'm kind of upset about. My dad bought our lessons (we took them together) and when our instructor told us about boots, he was like "Ok we'll just get the minimum for now to save money." which was fine at the time, but when we got to the store, I was a good little girl and got my bottom-of-the-line, stay-on-your-horse type of boot. Something like 110 dollars. (We agreed to NOT spend more than $100 each.) He took his dang sweet time and looked at the entire store, and ended up paying about 150 for Ariats that were marked down from 200. The thing that bugs me is that he NEVER rides and his boots are twice as good as mine. (Okay done whining.) New boots are high on my list of "stuff to replace" once I've saved up.
And Chella, what stretches did your teacher have you do? I've noticed my legs are way more tense than they ought to be, so that could very well be contributing.