When  a chiro or physio or even you own self stretch things back into place, it is only a temporary fix. Years of musclular habits have to be unlearned. If you observer yourself closely you might notice you always twist a little, maybe always cross the same leg over, always look more to one side than the other when working or watching tv, or just sitting. Look at your footsteps in the snow when walking or running, one foot may be placed differently. So to retrain, always make yourself sit straight, or if you've been sitting a little this way for  a while, sit the opposite way to counterbalance the habit. Also do exercises that use one side of the body, but then be sure to do the same exercise for the same amount of time and effort on the other side. You will notice one side considerably stronger, one weaker, so work both, and maybe the weak side a little more than the other, e.g . look up side plank exercise, do side leg raises. place a broom handle behind your neck and hold on with both hands equidistant and gently twist and bend to stretch both shoulders back. When you scoop horse manure, give both sides of your body some work, but build the weaker side gradually. Shovelling pulls the muscles forward, you need to counter balance with exercises that pull the shoulders back like touching your shoulder blades together.   Don't neglect core exercises, because if your core is weaker on one side (or even both sides like most people) then every other muscle has to try to compensate. Even your horse may become lopsided trying to compensate for your crookedness. There's no quick fix,it's like dieting, you have to maintain your improvements with daily (even 15 minutes a day can make a big improvement) strengthening exercises.                        

Chiropractors like to straighten out tight muscles but will rarely show you an exercise to fix the problem because they prefer repeat business. If a chiro hasn't properly fixed the problem in a couple of weeks, what is it doing for you?

Pilates is great for fixing posture problems. Pilates work on a ball is especailly helpful

All excellent points above :) One thing that I always reiterate to my students is "make your collarbones as wide as you can" and "lift your collarbones to the sky." This usually gets them to drop their shoulders and relax the tricep (which clenches against the body when shoulders are tight and rounded), giving a softer, more forgiving hand and a more elastic horse.

I would also recommend getting a pair of 5 lb dumb bells and a resistance band. With a dumb bell in each hand, hinge over at the hips so that your torso is parallel to the floor. Stick your bum out and pull your belly button into your spine; you want to support your back as best you can; never round it out or you'll pull it for sure! Now that you're bent over, let your arms hang toward the floor. Feel how gravity wants to pull your shoulder blades apart and round your shoulders forward over your chest? Pull both scapula back so they lay flat on the back of your ribcage. Now, with a slight bend in your elbows, lift the weights out so that your arms are parallel to the floor, breathing out through your mouth as you do so. Return to start. Repeat for up to 15 repetitions, have a short rest and do it again, performing the exercise no more than 3 times a week. If you can't make it to 15, that's ok, do as many as you can WITHOUT compromising your form.

I have worked my way up to 4 sets of 10 reps with 15 lb dumb bells, so I can personally vouch for the difference it makes in your posture! By training those rear deltoids (shoulder muscles) and trapezius (a triangular muscle in your upper back) it will help to lift your shoulders and draw them BACK, creating better posture and a more balanced body.

If you have trouble with the dumb bell exercise, put a hook into a wall at about head height (make sure it's in a stud and that it's a sturdy hook!) and hook your resistance band over it. You perform the same movement, but you can do it standing rather than bent over. :) Hope this is helpful!