Videoing is the best way, although it's not helpful in the moment. You have to know what you're looking at when you watch yourself though. It would basically be up to you to educate yourself on riding technique with books, videos, youtube, etc. and figure out how to apply it while you ride.
Video is a great tool. When I was in college, one of the girls on the team took tons of video during practices and it helped so much.
Whatever discipline you do, watch videos of the pros. Get on YouTube, watch as many videos as you can, study how they do it, what they're doing, and just basically absorb everything you can. I do this pretty much all the time even with a trainer!
If there are any shows near you, go and watch. You'll start to pick up what works, what doesn't work, etc. and watching in person offers you a different perspective than watching videos does.
Additionally, Judge My Ride is a great resource. Here's more about what they do:
"Judge My Ride offers a place where equestrians can upload videos and photos from their favorite horse shows or just schooling at home, and have them critiqued and commented on by international riders, trainers and Nationally rated judges in our sport. Our goal is to bring the global equestrian community closer – in a supportive, constructive environment." www.judgemyride.net
I haven't used them myself so I don't know much more than what they post on Facebook or Pinterest, but I've heard really good things about them.
I hope this helps you out!
Whatever type of riding you do, the more correctly you use your leg, seat and hand aids to communicate with your horse, the better he’ll perform. Many riders don’t even realize that they’re sending their horses confusing and contradictory signals.
That's why this is so important to improve, more and more. Listen to your horse, how he reacts, and you'll already see what you have to improve :)