In their book The Power of Full Engagement, Jim Loehr and Tony Schwartz describe a study in which a group of women agreed to do a breast self-exam during a period of 30 days. 100% of those who said where and when they were going to do it completed the exam. Only 53% of the others did.
In another study, drug addicts in withdrawal (can you find a more stressed-out population?) agreed to write an essay before 5 p.m. on a certain day. 80% of those who said when and where they would write the essay completed it. None of the others did.
From the Harvard School of Business blog.