Find a thin belt in the same color and material as your boots.
While wearing the boot, place the center of the belt at the lowest point on the back of the boot.
Wrap and crisscross the belt all the way up the boot shaft.
Secure the buckle as tight as possible. Voila!
Bring your boots to a professional cobbler. Wear the same socks you like to wear with your boots and have the cobbler take measurements with your pant legs tucked in to ensure that the boots are not sized down too much.