No one should ever underestimate the usefulness of just sitting still on a young horse. It is easy to overlook that they need to learn that just like they need to learn how to turn and stop. One of my favorite things to do is, right in the middle of a workout, I will just stop them and let them stand there still for quite a while, sometimes 10-20 minutes or more and just do things that you want them to stand still for. I'll move my legs, re-adjust my seat, move my hands and re-adjust my reins, jiggle the breast collar and back cinch, reach up and scratch them on the neck, reach back and scratch/pat them on the butt, move my weight back and forth and make the saddle move, etc. The last thing I want to happen is I send a horse home and find out the owner ended up falling off because they reached back to pet the horse's butt and it spooked from that or boogered sideways when they leaned forward to pat them on the neck.
When I want them to move forward, I make sure that I am very clear about the forward cue; equal leg pressure on both sides, a forward seat, and I will move my rein hand forward closer to their head on green horses for the first few times until they are solid on just legs and seat. There are very few things that get on my nerves quicker than a horse that just won't stand still.
Sometimes they don't want to stand still, that's okay. If they walk forward, I'll turn them in little circles 'til their feet stop or back them up a few steps, then I let them stop again. It really helps if they are pretty worn out first because, let's face it, most green horses have attention issues right when you first get on anyway and asking them to just stand there and be still for an extended period of time is about like asking a kid for the same thing at a birthday party, haha. Of course, they need to stand until you ask them to move, but if you keep the first few sessions short, then you are setting them up for success.
My favorite time to do it is right in the middle of loping/trotting circles. That way, they can learn that sometimes they will be required to go from higher speed to a standstill and getting to just stand there and breathe for a minute is nice. Horses that appreciate the moments of stillness are ones that don't prance around, even when other horses are milling around. They may keep an eye on all the activity, but they will keep their feet still.
In my experience, that is the one thing that most people want and so few of them have; a horse that is content to just stand there while you talk to a friend or watch a rodeo event. Or, even more importantly, stand still while you mount and get comfortable and when you dismount.