I did this a long time before I started reading lesswrong, the idea of doing it as an organized, self experimentation type thing didn't even occur to me.
This was about five years ago so I can't offer you much detail. I quit flat out, cold turkey. I think the gagging thing was most helpful in the initial two weeks when intense cravings where a problem particularly at parties etc. where people were smoking around me. I remembered to induce gagging I would guess about 1/3 to 1/2 of the time I consciously experienced a craving, which would have been dozens of times per day for the first week and far less over time. I did it whenever i caught myself thinking in any real detail about cigarettes, the taste, etc etc. One of my main reasons for quitting was were the queasy feelings upon waking - I'd remind myself of those general disgusted feelings, recalling as vividly as possible the unpleasant feeling upon waking up etc.
I have no idea which of the various things i did contributed to my success. I suspect developing the habit of physical exercise has been a big help because I find it impossible to run, swim or box if I have smoked in even the last week. I did relapse for a few months about two years later.
Instant uncontrollable vomiting definitely wasn't a problem. I have a fairly low gag reflex, so I could gag a little to the point where I felt a slight response in my eyes and stomach muscles without worrying about actually vomiting, although for all I know this might increase the effectiveness of the technique.
My biggest caveat is that I had been truly addicted for at most a year and a half. It might not work so well for someone who's been on the death sticks for decades. Particularly if it's been entrenched into their routines.
Ok, so you didn’t smoke-then-gag, just gagged when you thought about smoking. Thank you.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.