We all know the scenario - you buy the girlfriend a bike, you spend a day servicing a friend's bike, you give an old bike to someone in the family or you spend time in the LBS helping a friend get a bike. Your motivation is out of genuine enthusiasm and in the hope that they will enjoy cycling, maybe to become a life-long cyclist.
As time goes by you realise that they have not really taken to the bike, it remains in the hallway/garage/shed and they still get around the old-fashioned way - walking, catching the bus or killing the planet with their noisy, cramped tin-box.
Although you daren't say it, you might as well have not bothered. They are not going to go cycling any more than a leopard is going to change his spots. No amount of logic or reason will change matters.
...then there are some people you advise on a new bike choice that do get into cycling. Although they probably did not need you to help them get cycling, you still have played some part through 'setting an example'.
In other walks of life it is possible to have success at introducing a newbie to 'something better'. For instance, where you can find someone wanting help with their badly rootkitted computer you can take all day to pull out the viruses or spend half an hour with the Linux install disk. Invariably they don't have a copy of Winmare 7 or find any signs they have joined the Mapple Cult next time you see them - they have accepted a big change despite having to learn-new-stuff (which is hard).
Alcoholics Anonymous have '12 steps' structure and a dose of religion to wean people off drink. Ultimately A.A. face a similar problem to the lost-cause-of-cycling in that people are not going to change unless they want to, however structure works for some A.A. punters, in some cases, some of the time.
Clearly this is an open ended question with many answers. Personal anecdotes are welcomed, did a childhood gift of a bike, a bike-to-work scheme or the advice of a doctor work for you? Is there anything that we can learn from other walks of life?
Statistics and studies could also provide answers. Whatever hints and tips you have to help people take up cycling as a way of life are welcomed. Answers with lateral thinking, tips from other walks of life are also welcomed.
Please mod up the better, more insightful answers.