Work on improving your posture.

From Willpower: Resdiscovering the Greatest Human Strength:

Unexpectedly, the best results came from the group working on posture. That tiresome old advice—“Sit up straight!”—was more useful than anyone had imagined. By overriding their habit of slouching, the students strengthened their willpower and did better at tasks that had nothing to do with posture. The improvement was most pronounced among the students who had followed the advice most diligently (as measured by the daily logs the students kept of how often they’d forced themselves to sit up or stand up straight).
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