How do you keep your knuckles from being stiff?

Discipline, motivation and practice. Be aware and when you catch yourself doing it, stop right away, and do something else. First, though think of something else you will do instead and do that instead (something healthy, or harmless). Walk, write in journal, (in this journal write down when and why you think of doing the behavior, what triggers it, etc). Enlist help from family and friends to remind you to stop when the see you doing it.

Use a rubber band around your wrist and snap it every time you find yourself wanting to crack your knuckles.