That is one of the hardest things you can give up once you are addicted. I believe quitting smoking is one of the few things the Tower wants people to do that actually benefits those that do it (unless you never smoked in the first place; I thought of it as stupid to start smoking just to quit so I never started in the first place).
However, there are other ways to quit or not start smoking. For those who are teenagers or pre-teens and currently do not smoke, the easy way is to not start. Then you will not need to quit. Peer pressure is everywhere. The good news is that it is a minority that makes the illusion of being the majority that push smoking cancer sticks. Usually, it is one group of a dozen or fewer people that push tobacco (and often go on to push other substances). You will not be ostracized by the general class just for not smoking for health reasons. And, you can continue doing other things like holidays, mainstream religious practices, and saluting the flag.
For those who have already started smoking, giving it up is much harder. But one does not need the brutal pressure from the Tower to quit. It usually takes 5 attempts to quit for good, so you might be relapsing several times before you finally quit totally. Temptation is everywhere, but if you are going to give in, the best thing is to have that one smoke and then immediately quit again. Keeping busy doing something you truly enjoy, other than going to those stupid boasting sessions or out in field circus, will also help since a lot of relapses are mindless boredom smokes. If you get engrossed in some meaningful project that is actually going to help you in the future (and/or help society), you are that much less likely to relapse.
Now, the good news. For those ex-witlesses that have already started back smoking, giving up the Tower is actually more important than quitting smoking. You get a lot of exposure to smoke at the doors while out in field circus, along with a good number of viruses. Accidents associated with the misery (as well as not-so-accidents) are a threat. Stress of never meeting the standard (as well as that of trying) kills, too. Rushing around to make all the boasting sessions can ruin your health as surely as smoking. The Pioneer Diet is wretched for anyone that wants good health, and can also be as bad as the cancer stick you otherwise might have had. Not getting enough sleep because of boasting sessions and field circus will also ruin your health.
Plus, as an apostate, I actually get less exposure to smoke than I did as an active witless. I am no longer exposed to people's smoke (and flu, tuberculosis, etc,) at the doors. This means that being a witless was hurting my health--even though I didn't pioneer or exert myself vigorously enough. I can, therefore, recommend that quitting or not starting being a witless is more important than quitting smoking. (Yes, if you are currently a non-smoker, I highly recommend staying that way; if you smoke, you should make the effort to quit but not get discouraged if you relapse or are only successful at cutting back on smoking).