It's amazing how positive, negative statements can be. Double negatives intensify the negation so much that they create positive action.
- Never quit quitting
- Never say never
Conversely, it's amazing how negative, positive words can be. Double positives intensify positivity so much that they overstretch limitations and have deleterious effects.
- Keep pushing
- Keep on keeping on
Thoughts predetermine action and therefore, willpower. But really all that's needed is good practice so you don’t have to think at all. What works is the execution, not necessarily the right frame of mind.