I'm just learning sitting trot, and my coach tells me to anchor my butt in the saddle and turn my toes in, but keep my feet level. It also helps to put all your weight on the balls of your feet, and turn them in as much as possible. And most important - sit back! I find myself leaning forward too often and it throws the balance off. Also, don't rush right in to doing full laps of sitting trot; start on a 20m circle (perhaps being lunged) and sit 2 beats, then post, then sit more, then post, etc... until you can sit quite a few beats comfortably. And do some canter before you start sitting because it helps stretch and loosen your horses back.

Hope that helped! :)

You just have to remember to keep your body loose, especially your pelvis. As soon as you tense up or tighten your legs you confuse the horse and lose the 'proper' position. It may look bad, but don't worry, you'll overcome it! For my first while I was getting air time between my butt and the saddle! I haven't got it perfect yet, but I've come a long way.

Try doing lots of canter-trot and trot-canter transitions; it should keep your body loose. :)

Throw in a few posts in between. Think of it as a rhythm and that your hips and knees are shock absorbers. Remember to breathe- it so helps.

I would ask the rider to do a lot of Pilates, especially Pilates on the ball. If you are are at stiff in your hips/lower back and hamstrings you will have problems.

For the average rider you start by doing 2 sitting and 2 posts at the trot. Over a period of time you can lengthen this out to 3 sitting, then 4, then 5 and so on. Do not make the mistake of slowing the horse down to make life easier for the rider.

a big part of the sitting trot in my opinion is how the horse goes - if the horse is over the back the sitting trot is just so much easier. 

so learning the sitting trot on a dressage school master really helps if you have the opportunity.

I used to be good at sitting trot, came natural, but then I took 2 months off and completely ruined all my work...what helped me regain my balance and motion was a used a western saddle. In order to feel comfy in a western saddle you NEED to sit back, so I used that and after about 10 minutes I was able to sit the trot good. It also fixed my main problem with cantering. After having a steady sitting trot, then going into the canter, I was steady, and able to sit the canter(unless the horse picks up the wrong lead, that throws me off balance.)

So if it's possible, switch saddles. They don't need to know western, just use the saddle to help them find there balance. (I ride English, but have had some informal western schooling. My friend has showed me a bit when we exercise her cousins horse and in lessons I've picked up a few things, like when starting the cool down this week, I asked my instructor to show me how to hold the reins for neck reining since the horse I was on knew it, and I figured out long ago that he responds to the reins on his neck)

Great tips from everybody! I'd like to toss in, no stirrup work might help as well

Yes, holding the pommel for a few strides is a great idea for getting the feel. Also, if you can do a few strides with your legs up and away from the saddle, you'll be able to find you balance and really get your seat bones down.

Sitting the trot and staying quite in your legs and body is hard though. I'm always working on it.

Hi,  there is great book I read called "An Anatomy of Riding" by Volker Schusdziarra and Heinrich Schudziarra.  It goes into detail about the hows and whys, and what muscles are used in riding.  Great book!! improved my seat and my understanding of riding.

Happy riding

If we're going with books that will teach you a lot about how to become aware of your different body parts while riding in order to become more secure and centered, Centered Riding by Sally Swift is always a great place to start. There is now a Centered Riding 2, as well as two videos, if you're interested in checking out her work further. http://www.centeredriding.org can tell you more.