Day 7 of Kelly McGonigal's "Boost Your Willpower" program, and she says:
To set yourself up for success, envision tomorrow as an extension of today—and as the first of many tomorrows to come. Assume that every decision you make today, you'll make tomorrow, too. And the next day, and the next day. Begin to view every choice as a commitment you're making to all future choices. When you can do this, it becomes clear how just "one more smoke" or "one more cookie" can add up over time. It becomes harder to deny the health and personal consequences of your actions.