The theory is if you practice them enough they become a behaviour, then they stick. So how much is enough?
Some people say you need to repeat something over a period of 21 days for it to become a habit (not constantly), others say three months. From my experience you need to consciously think, act and do the new activity over at least 21 days, then be conscious that you are practicing it for the next 2-3 months. Theres a good chance it'll stick by then or be gone if you're trying to break a habit.
The really hard bit is thinking 'what new habit do I want to do'
One for me in 2012 is:
" Each day I have dealt with everything in my email in-box"
Now I have gone public with it, so I have to do it.
Any you'd like to share with us?