I guess it depends on where you are trying to place your will power. If it's on something you have little value for, then it's better to find the value, or let it go.
If it's something imposed from the outside, then your chances of conquering it are pretty low or nul.
What I try to do is look deeply into my heart to see what is really important. Then I vision myself having conquered it. Now I've fueled my willpower with my heart's desire and holding on is much easier and effortless. If I do fall off the wagon, I can more quickly pick myself up again.