Riding without stirrups every time you ride will help a lot to keep your lower leg stronger and stiller. Even if you only ride without stirrups for even 10 or 15 minutes every time you ride, you will see a big improvement! But the more often and longer you ride without stirrups, the stronger and stiller your leg will become.
In trot and canter try to hold yourself out of the saddle for a lap or even a longside of the arena/paddock, making sure to stretch your heels down and to keep your own balance (hold your horses neck if needed, but idealistically you should be able to stay standing without doing that)
You could ride without stirrups
Standing up in your stirrups