Supposedly our subconscious cannot understand negatives. Therefore we should formulate all our goals in a positive way. Instead of writing down "Don't smoke" as a goal we should use "Be smoke free".
How much merit is there to this idea?
Here are some sources to show that this is a notable claim:
Most self help subliminal programs are designed to get you to stop an unwanted behavior, but unfortunately the programmers either don't know, or don't care, that the subconscious brain cannot process the negative words in these programs, as such if they tell you not to chew your nails, for example, all your subconscious mind is aware of, is the thought of chewing your nails.
This can also be found in some books: