Another positive reason for not smoking or being exposed to secondhand smoke in pregnancy - you are much less likely to be at risk of a premature birth, which take it from me, is a nightmare you do not want to experience.
I'm an ex-smoker, some things that worked for me are:
Taking note of the situations where the desire for a cigarette arises. It could be after a meal, when your mother visits (!) etc etc. Then you need to break your behaviour so that you do something else at these times to distract yourself - go for a walk, ring a friend for example.
Wearing a rubber band on the wrist and snapping it against my skin when I felt like I was reaching for a fag.
Remembering that with every single day without fags the amount of cravings you have to endure will decrease. It may not feel like it at the time, but they will. And each craving does not last for ever, they rise and then they fall away, until the next one hits. The spaces between them get bigger and bigger very quickly.
Remembering that you will feel shtty, emotionally and physically, as your body rids itself of the toxins. Exactly how shtty you feel is directly related to the amount of poisons you have put in there. Remembering, it's the toxins coming out that is doing it and leaving a happier, healthier body behind.
Nicotine is a massively powerful addiction, second only to heroin I believe. Do not under-estimate the power of it to draw you back in. It will twist your mind to make you feel that "one is OK", it's not. Look at it as a big monster that sits on your shoulder whispering in your ear. It feeds by draining life force from you by making you smoke. The only way you get rid of the monster is to cut off the life force by not smoking. If you stick at it, the monster gets bored and hungry and goes away but not before it's put up a big fight.
If you do fall off the wagon, don't beat yourself up. Just strengthen your resolve, think of your baby and start again.
Massive amounts of luck.x