As mentioned in Slackercruster's thread "What are your biggest photography problems/challenges?", several posters listed portraits and shooting strangers.
There's an easy way to overcome this hurdle, at little risk to yourself;
Public Events.
Take your camera and a few lenses to any rally, festival, show, protest, demonstration, live performance (if cameras are allowed), carnival, fair, expo, street music or other (usually "Free") public event.
The participants are used to cameras, that's why they're there. They will willingly pose, not just for you, but virtually anyone with a camera. They're not always looking directly are your lens; Other are taking advantage of the "Photo Op".
Here's a few examples from a Denver Bronco Rally, Car Show, street musicians and Cinco De Mayo.
It's fun, great practice and safe. Try it!
Ron