Affirmations can be helpful for a variety of purposes, but they are especially effective in helping ease the discomfort of quitting smoking (or eliminating any bad habit).  Why?  Because your thoughts usually determine how you feel, and your feelings about something usually trigger corresponding actions.
One of the reasons why smoking is seductive and pleasurable for many people is because they keep telling themselves that they enjoy it!  Think about that for a moment.  They tell themselves constantly that they love smoking, they need to smoke, they want to smoke.  So they smoke.
This same concept can be used in reverse – to get yourself thinking and feeling that you DON’T want to smoke.  However, you don’t want to insert references to smoking in your affirmations, simply because you already have a belief that smoking is pleasurable.  Once that connection has been made in your subconscious mind, any thought of smoking is going to trigger a strong desire to smoke.
Instead, you can use affirmations to emphasize the benefits of not smoking – without mentioning smoking at all.
Here are some examples:
I love being able to breathe freely.
I love taking good care of my body.
I nourish my body with healthy habits.
I deserve a clean, healthy body.
My teeth are white, my skin is glowing and I feel great!
I can change my habits by changing my mind.
I honor and respect myself always.
I am strong enough to overcome any challenge.
I feel so free.
I take one step at a time to where I want to be.
I trust my inner guidance to lead me to a state of perfect well-being.
I believe in my ability to do anything I desire.
How to Use Affirmations
Affirmations work best when they are recited repeatedly and while giving your full focus to them.  Not only should you say the words, but you should also do your best to call up the corresponding FEELING associated with the words.  For example, if you say, “I feel so strong and empowered” you should actually make an effort to feel that way.  This does take practice if you are not used to controlling your emotional state, but it gets easier the more you do it.
Constant repetition many times a day is important also, because you are attempting to override existing beliefs in your subconscious mind.  A belief is nothing more than a thought you have thought many, many times before, until eventually it becomes your “truth”.
For example, the belief that smoking calms you down.  Physiologically, smoking does not have relaxation benefits but many smokers have convinced themselves that it does.  Because they believe this, they seem to feel more relaxed after smoking.  To override this belief, you need to replace it with an opposing belief – like affirming that you feel calm and relaxed already, so there is no need to smoke in order to relax.
The example affirmations above will help get you started, but feel free to write your own too!  Think about the aspects of smoking you enjoy, and come up with a phrase that affirms you already feel that way (again, not mentioning smoking!).  Likewise, think about how you feel as a non-smoker and emphasize those positive qualities too.
Affirmations do take time to fully sink in to your subconscious mind, but just like your old beliefs were formed, consistent repetition and reinforcement will help them become permanent.