I found this on a website on how to correct the chair seat (How To Improve Your Classical Seat In Horse Riding (Horse Riding))

It gives some good exercises.

Correcting the chair position

1. One of the most common problems is known as the 'chair position'. This is when the lower leg comes forward. All lower leg problems come from having an insecure seat. This position causes problems as you can easily get 'behind' the movement of your horse which causes you to tip backwards and grip up with your legs. Riders with this position have too much weight on their seat bones.
   
   Solutions:
   
   CAUTION! As a general caution do not attempt to do any of the following exercises if you suspect a lower back problem. Consult your doctor first.
   
   
2. Firstly, take away the stirrups by crossing them over the saddle in front of you. Then wiggle around in the saddle to feel all the three points, the seat bones and pubic bone.
   
   
3. Rock forwards and backwards until you find a comfortable place and the pelvis feels upright.
   
   
4. Circle the ankles towards the horse. This exercise really helps relax the hip at the top.
   
   
5. Try swinging one leg forwards and the other backwards, keeping the toes in and the heels out, so as not to kick your horse.
   
   
6. Hold the front of the saddle with one hand and with the other grab an ankle and stretch your knee to the floor. This strengthens and opens the hips and pelvis. Hold this position for thirty seconds. Relax and then repeat with your other leg.
   
   
7. Here's another exercise to try. Hold both reins in one hand, bring both legs up to the top of the saddle and then push both legs back down towards the hocks of the horse, without tipping forward! This will stretch your hip flexors. Relax and repeat this exercise several times.

 

Great post Erika, but I would also like to add, a saddle can also be the cause of this position.

I've been in certain types of saddles, that do not aid my position at all, and instead, hinder me, like putting me in a "chair Seat". No matter how hard I tried to correct it, I could not, it was a big struggle to get my legs under me, and sometimes even physically hurt - so finding the right saddle that works great for your body type, is very important as well.

I am not saying that the saddle is always the "excuse" for the problem, because yes, a lot of the time the rider is just not being taught or shown correctly, or corrected - but what I am saying is, it isn't always the rider.

 

It can totally be the fault of the saddle. Either the distance from the balance point of the seat (where your seatbones rest) to the stirrup bar (where the fender/stirrup leather hangs down from) is too much
or
the saddle is angled up in front due to it being too narrow for that horse's shoulders and so it causes the rider to be forced to lean forward with their upper body to compensate. when your upper body leans forward, it becomes necessary for you to also move your lower leg forward to counter it, thus you become in the dreaded "chair seat".

The Chair seat is bad not only because is puts too much pressure straight down under the seat bone (none of the rider's weight carried over the thigh or pubic area) BUT it encourages the rider to BRACE into the stirrup, pushing their bum into the cantle and their foot into the stirrup, effectively locking themselves in place and against the horse's motion. Makes forwarm motion harder for the horse because it must work against a rigid, blocked lower body, which often results in too much movement in the upper body of the rider.

 

Ok, I don't think I have the issue in the saddle if my stirrups are long enough. At least if i do, it's not nearly as bad as bareback. I have noticed myself sitting funny that way. I did not used to do this. I do it while trotting. Is this a pretty typical problem to acquire while sitting the trot bareback?