I tried an experiment on Sunday. I was feeling a bit slothful and grody from eating nothing but junk food on the previous day, so I pulled the stirrups off the saddle. I didn't ride for long - maybe about 25 minutes - but it was good. Really good.
So I mentioned it to L. in my lesson on Tuesday, and pulled them for the last 20 minutes of my lesson, and she agreed. Forcing myself to ride without stirrups dramatically improved my balance and seat in the canter, and consequently made him straighten up and pay attention to my aids better. From now on, non-lesson rides will be sans stirrups, and I'll drop my stirrups for the end of the lesson until I can drop them for the entire lesson. It'll be good.
My other takeaway from last night was, once again, that I should resort to leg instead of rein when he gets hard in the bridle. We had a long, intense, frustrating conversation about left bend last night, and the only time I made headway was when I stopped thinking of it as a hard against the rein problem and thought of it as a resistance to leg problem.
In a similar vein, I need to work on my application of leg aids. I have a bad habit of inching my legs up even slightly to cue an aid, and I need to think more long and wrapped around instead of bringing them up to put them on. Granted, when my legs stretch all the way long my heels are below his barrel, but still. If I have to bring them up - even slightly - to put a spur on, they need to go back immediately.
Riding Thursday, he'll get jumped by a lesson kid on Sunday, and then maybe, perhaps, hauling somewhere on Monday? Fingers crossed the weather holds up.