Sooooo here it goes. Think about riding, and how tense you can get up there. Your shoulders, your arms are locked, you aren't supple, and your horse can't possibly be if you aren't. What to do?
Try this: Where you are sitting right now, think about your lower back. Is it tense? Chances are pretty good, that yes, it is.
So. Now, think about relaxing your lower back. Do so, and breathe, and feel the effects that come from simply relaxing your lower back. Your whole body will release.
It starts at your lower back and travels to your shoulders, your arms, your elbows, and your wrists. Once your shoulders and arms are unlocked, your neck can relax, followed by your jaw. Your butt, your legs, your ankles, your feet. Somehow, everything can relax simultaneously if you are able to fully relax your lower back. It's great! Try it in the car, at work, and at your computer desk at home. It is very bizarre.
It is even weirder when you transfer that exercise to riding. It is very cool! How can you expect to move all of your limbs independently of each other when they are tense? If you can relax, your horse can relax. If you are tense, your horse will feed off of that and things can snowball quickly.
Try it next time you ride. Get on your horse, and from the second you get on, think about your lower back. Relax it, and continue on. Work towards a relaxed lower back for every minute you are riding. Try it on your transitions, while you are loping, anything. It is a fun, but difficult task to master, but holy cow it can do amazing things for your posture and the way you ride.