How do humans actually change?

In the 1980s, a psychologist named James Prochaska developed the transtheoretical model of behavior change (TTM) based on research showing that people generally don't "just do it"  as Nike (or a new year's resolution), but instead tend to move through a series of sequential stages that look like this:

1. Stage 1: Pre-contemplation (Denial/not realizing you have a problem)
2. Stage 2: Contemplation (rife with ambivalence, shows up as resistance even though you might be willing to talk about it. Just can't get yourself to act. This is important because this stage is of procrastination - reluctant to give something up without knowing what they'll get in its place).
3. Stage 3: Preparation 
4. Stage 4: Action
5. Stage 5: Maintenance