Very shallow serpentines. They're fantastic for establishing to inside leg to outside rein connection and create the subtle bend in the body that you need for proper flexion. To do shallow serpentines, pick up a nice walk, establish proper rein contact being sure to follow the horse's motion to keep the contact consistent. To start the exercise, use an indirect inside rein by rotating your hand counter-clockwise a quarter turn so your knuckes are facing up. While you're doing that, just take your inside knee off the saddle so your horse bends in that direction. Just take a few steps in that direction, take one straight step, and then do it the other way. The other way you can use your legs in this exercise is the more proper way, but the exercise I learned just involved the above method which is why I posted it first. The more proper way to use your legs are, while applying the indirect rein, soften you inside leg so your horse has a nice soft object to bend around. Bring your outside leg back a little and use your outside rein to cue him to bend off the rail. I actually like the first way better because it is extremely simple and still accomplishes the same result. You're not looking for any extreme bend, just enough to get the horse to "wander" in the direction you want. The head should only bend enough to where you see his inside eyelash and nostril. Coincidentally, it's the same degree of flexion required to keep a horse supple and moving freely. Sorry this is so wordy. Hope it makes sense.
You just have to see your distance...you don't have to like it.