Ah, the lower leg. It's too tight, it's too loose, it's too far forward, it's slipping back, it's everything besides what it's supposed to be: Soft and draping, so that the heel is in line with the hip. In the trot and canter, the lower leg should "breathe" against the horse, neither flapping against them nor locked stiffly in place. Toes should be forward. Sigh.
If we're too stiff, we can't use our legs properly to give cues. If we're too loose, we'll feel insecure and will likely stiffen up elsewhere to compensate. A noisy leg might desensitize the horse to the leg aids over time. A locked leg doesn't communicate anything at all, except tension. A foot that flaps around too much might jab the horse with the spurs accidentally. A too-stiff ankle will limit the fluid use of the leg aids. Jamming into the heels too much can contribute to the lower leg being too far out in front and puts the rider in a "chair seat". Not having the heels down enough can make the leg feel shortened and the hip flexors tense. Toes that turn out too much cause the rider to ride on the backs of the legs too much, the knees to pop off the saddle, and contributes to "chair seat". Turning the toes in encourages pinching with the knee, which pops the lower leg off the horse and the seat out of the saddle, encouraging an unbalanced pitched-forward position in the upper body.
...Have I said something yet that sounds familiar? Nearly every rider grapples with one or many of these issues of technique, no matter what discipline they ride, at some point or another. Personally, my feet (especially my right foot) like to point outward. They do this naturally, even when I am walking, so it is a physiological asymmetry rather than a bad equitation habit–which makes it much harder to correct. I have been doing yoga and getting chiropractic adjustments to help the issue, and I'm also trying to be very aware of it while I ride because it is a big reason why I lose my leg out from under me, most notably at the sitting trot, and end up in that dreaded chair seat.
One little mental trick that works for me is to imagine I am touching my toes under my horse for a second. Of course I can't really do that, but if I imagine I'm doing it, my toes turn forward, my lower leg stays back where it should, and my core muscles have to "turn on" to do the work of stabilizing rather than relying on my hamstrings. I use this imagery when I first start out in sitting trot, to get myself into the correct alignment, and once I'm in that place of balance and correctly using my core, I can relax and just ride.
Another common problem is the "jammed down" heel and very stiff lower leg. I see students doing this all the time, especially when riding spooky or excited horses. It may feel more secure to have the heels way, way down but actually it is making the low leg "ski" out in front of you, for which you then have to compensate somehow–usually by leaning forward too much and sticking the hips out behind you. This tense and perched position is not good for much and usually makes an already tense horse even more so.
To fix this, you must learn to trust your seat to be your anchor point, rather than the stirrups. Do some riding without stirrups on a quiet horse or on a lunge-line and sit as "deep" as possible in the saddle, feeling how your stomach and back muscles have to work harder when you anchor through your seat. Let the legs drape against the horse, long and relaxed–don't "hang on" with them–your seat and core are what keep you on the horse.
When this feels good at the walk, see if you can maintain that position and feeling at a slow trot (it helps to hold a bucking strap at first so you aren't tempted to balance on the reins. I used to hold the straps on my saddle pad because I didn't have a bucking strap on my saddle. You can keep your reins in your hands at the same time.) Again, don't grab with the legs–try to let them "breathe" against the horse's sides. Don't let the upper body tip forward. Keep the "back pockets" in the saddle and imagine the pelvis rotating up and down with the rise and fall of the trot. I like to imagine I have a big fancy belt buckle on and it tilts up, up, up with the "up" part of the trot. You can't be stiff in your lower back, and you can't be hanging on with you legs or else your seat will lighten and you'll pitch forward and "bounce" against the rhythm.
Now, once that feels good, take your stirrups back but maintain the draped, soft feeling in the legs, the engaged "working hard" feeling in the core, and your deeply connected seat (sit bones might even feel bruised after riding this way–that's a good sign!) You no longer need to grab with your leg and jam those heels down for security because you can trust your own balance and core strength!
Do you have a particular "lower leg struggle"? Have you figured out a trick for fixing it?