I had made multiple attempts to quit before and once again failed during the summer of 2014.
I was upset over that and I was going to turn 39 that October. It occurred to me that 40 was right around the corner. I had said that I was going to quit when....I graduated college, I turned 25, I turned 30, etc....and here I was still a smoker in my late 30's and I knew I had to quit soon. If I hadn't already done irreversible damage to my body, it wasn't going to be long. (I already had a horrible smokers cough, was always tired, depressed, etc.)
Instead of jumping into another quit attempt, I decided to educate myself by looking for resources on the internet. I stumbled across QSMB (another quit site that no longer exists but many of us are from there), joined, and the support and knowledge I got there helped me quit and stay quit for good.
I think support and knowledge is what helped a lot of us which is why many long term quitters are still here.
So, I quit cold turkey with the help of online support and knowledge (not to mention Allen Carr's book which I would recommend to anyone trying to quit).