Self-modification through planned peer pressure

I recently had the thought that one of the most effective ways of helping yourself change for the better may be to surround yourself with people who are similar to who you aspire to be. 

People often want to change their habits but sometimes lack the willpower to do it.  However, people are innately social creatures.  Having other people of a particular type around you uses the "peer pressure" that overprotective adults warned us about as kids, but harnessed for a self-chosen purpose.   Want to exercise more?  Hang out with people who like to exercise.  Want to learn to be more optimistic?  Hang out with optimists.  Even if the pressure is not overt, the presence of the other people will at least provide constant mental reminders of the habit you want to change as well as a positive reinforcement structure for making the change. 

I'm wondering what kind of research there is to back up or refute this.