Nutrition - changing a habit

Apparently it takes 21 days to change a habit (I have no idea where this fact came from but I have heard it 100 times so it must be correct right?)
I am thinking that to break the many unhealthy habits I have in the nutrition department, I should spend at least 21 days doing something fairly strict.  My friend Sheena and I made a plan today, and for the next month we are committing to a cleanse together. We are thinking that the accountability to each other will be helpful to get me on the right path and help Sheena eliminate anything about her body that reminds her she recently had a baby.
I will write the details of the cleanse coming up, but basically we just committing to a from-the-earth sugar free diet for 30 days. I am going to suck up all of the crazy cravings for a month (please forgive the inevitable mood swings) and then reassess....I'm hoping that it will be long enough that continuing with a healthy diet (less extreme, but very healthy) will be less of an issue, but I'm just taking this month for now.