Here's some handy advice and an exercise to keep those heels down.
I know that everyone has always told you to get your heels lower. You've probably been told that you have to drop your heels so that you can have better balance and contact with your horse's side. They've said that the longer leg stabilizes your balance and gives better aids.
So we grin and bear it. Despite the discomfort, we push those heels down. We grunt and groan while we try to keep the heel down through the transitions, bends, and canters. We do what we have to do to make it look good.
Why We Shouldn't Force the Heels Down
Some of us have an easy time getting the heels down. If you are one of those people, you will wonder why the rest of us have to work so hard at it. For other people, overall body tightness plays a factor in how they can release through the legs.
When you push down, you drive tension into your leg. Invariably, the tightness in the heels cause the knees to pinch on the saddle. The knees cause tightness in the thighs and then you find your seat has an uncontrollable tendency to bounce against the horse's movement.
Aside from the effects on your body and position, you also affect the horse. The tight knees prevent the horse from moving freely and might contribute to sluggishness in the horse's movement, reluctance to swing through the back and in the long term, even gait abnormalities.
There is no way to force your heel down without causing some sort of unwanted result. The tension in your heels can transfer all the way up the leg and into your seat.
What To Do Instead
In order to get your heels down the way we see in the equitation books or by more advanced riders, you need to develop suppleness through your joints and tendons. This requires a long-term commitment to changing the way your body moves. There are several ways to train suppleness into your leg. Many activities can help - dancing, gymnastics, yoga - anything that helps to stretch and loosen and strengthen especially the legs.
If you are not the type to cross-train, you can work on the same thing by standing on the edge of a staircase. Hang your heel off the edge of the stair and let it lengthen so that it drops below your toes. Then stay there for a minute or so, just letting the joints and tendons learn to release in that position.
Once you are on the horse, the key is that the whole leg has to stretch - right from the hips. The hips release, the knees soften and the calves sit even closer to the horse's side. Only then will the heels stretch below the toes - all on their own. It's not good enough to just push those heels down.
When you first get the "real" stretch, it feels incredible. The leg really truly becomes long and you feel like you've wrapped your legs right around the horse in a wonderful bear-hug. The hips open enough to let the legs dangle down so that the legs and seat seem to just flow effortlessly along with the horse's movements. There is less struggle to stay with the horse because you supple into the horse. The best part is that your ankles just naturally "drop"- in the sense that they couldn't possibly be anywhere other than below your toes. There is no force, no push, no positioning. It just is.
In the Meantime...
Riding more frequently will definitely help. Know that correctly dropped heels are a product of suppleness and length in the leg. Work on changing your body, not on just the appearance of your position.