I ride on my college's IHSA team and we have regionals coming up. I've qualified and I'm so excited to hopefully ride my best and advance on. One issue I've noticed lately is that my left leg tends to slip forward too far. I've just recently started riding again since a lower back injury, and I'm not sure if that has something to do with it. Does anyone have any tips to help me make sure I'm sitting square and even in the saddle?
What helps me is warming up with no stirrups and really thinking about your legs being pulled down into the arena sand. Another thing you can do is take the leg that tends to go forward, flex it up and back (so your knee is pointing down) and hold it up by your ankle. It will stretch and open up your pelvis so you can stay in position. Just don't overdue it.
And you're sure the saddle isn't playing a part in it? It could be asymmetrically 'level' like one of your seat bones is slightly higher up than your other one.
Also, some work in two point gets your legs underneath you.
I ride IHSA too (good luck at regionals!) and I know all too well you can get stuck with some pretty crap-tastic saddles and sometimes you just can't help what they do to your body and position. Is it something that also happens in your lessons? It may just be that you need to get your muscle memory back after your little break. Just keep working at it. Some no stirrup work might be beneficial.