My opinion is that will power is to be used only in the short term, like a few hours or a couple days. Use will power strategically to make changes in your environment.
For the long term, I think internal motivation works much better. Why do you want to do X? How badly do you want to do X? Does the motivation truly come from inside, or did you choose it because you "should"? (For example: I "should" audition for orchestras, but I teach lessons because I feel like I'm making a positive difference in the lives of my students.)
With habits like cracking knuckles (or looking in a mirror), there's almost always some sort of internal reason we picked up the habit. If you deal with the internal reason first, it becomes easier to undo the habit. I have found that choosing not to crack my knuckles in the morning is much more successful than telling myself I will never do it again.
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