Knowing you do not want to smoke and having something to actually say in the moment are two different skills. Social situations move fast, and hesitating even for a second can be enough to say yes out of habit rather than choice.
Having a few real phrases ready in advance takes the pressure off having to think of one on the spot.
For a simple, low-friction no. "No thanks, I'm good." Said casually, without extra explanation, this ends the moment quickly for people who are not going to push further anyway.
For someone who knows you used to smoke. "I actually quit, so I'm sitting this one out." This is honest and specific enough that most people will not press you on it.
For someone who pushes back with "just one won't hurt." "I know, but I'd rather not start that up again." This acknowledges what they said without arguing the point, and repeats your position instead of defending it.
For a work or social event where you want to change the subject fast. "Not for me, but go ahead." This gives the other person permission without making your choice the center of the conversation.
For a moment when you are genuinely tempted and need an exit, not just a line. Step away physically, even briefly. "I'll catch up with you in a bit" buys you distance from the trigger, which matters more in that moment than the perfect wording.
The specific words matter less than having any of them ready. A rehearsed no is much easier to say than one you are inventing under pressure, and it gets easier every time you use it.
If you slip in a moment like this, it does not erase your progress. It is one moment, not a verdict. Get back to your plan at the next opportunity, not tomorrow, not after this event, the next chance you get.