I always figure that actions say more than words ever can.

People I know have seen me drop from 280-290 pounds and woefully unfit to more like 230 pounds and fitter than most of the people in my social circles.

If you live in a city sooner or later people will see for themselves that cycling is faster than driving, they'll see that it saves fuel and parking costs, they'll see benefits for themselves. Even if they are stuck in traffic and see cyclists buzzing past them, getting where they are going faster, they'll see real benefits.

I don't specifically try to convert people, although a few times people I know have expressed surprise when I've gone from the same point as them, to the same point as them, on my bike, and got there faster than they did in their car. At a stroke they know I'm not making up the numbers. But of course it takes effort to get to such a point and, until someone is willing to make the effort and accept that on day 1 cycling will be a slow, sweaty and tiring proposition, they aren't going to change.