The scene I’m about to paint is one every photographer whose spent any real time in a city or abroad should know well: you’re walking down the street, camera in hand. It’s a gorgeous day. Then it hits you – there, standing not twenty feet away is the perfect shot. A man. A place. A moment. You lean forward, camera-in-hand, and the hesitation strikes—you should probably ask the fellow first, after all.
But how best to go about it? Newbies will often state the predictable, a fear that transcends jobs, situations, and just about any other healthy human boundaries you can think of: rejection. People seem to fear rejection worse than death, and I suspect it ties in with that whole fear of public speaking deal. Others, perhaps a little more realistically, simply fear the fellow will see them coming, and promptly skitter off the other way.
It may sound like tough love, but the best advice on this is simply: get over it. Or more accurately, work through it. You’re right, they could reject you, but maybe it’s the writer in me talking when I say—rejection is just a part of the process. It’s part of life. It’s the worst they can do to you, and in the scope of things, it’s really not that bad. There’s other photo opportunities, and they will present themselves so long as you push yourself forward.
Keeping it practical though, my best advice on the approach would be this: don’t go jogging on over with your camera already primed and ready for the shot, clasped in your sweaty photographer hand (yes, I’m envisioning a hot, sweaty summer day. So shoot me.). Slide the camera to the side or behind your back, put it in your camera bag. Don’t get rid of it entirely, certainly, but just shuffle it out of the focus of your approach, and you will indeed be less “intimidating” if you so fear it.

Also: do not, hear me, do not go trotting up to a child with a photo in mind before first approaching their parent. This can get you in heaps of trouble. The child telling you a photo’s alright is not good enough—get parental permission. Heck, given how much parents love pictures of their children, too, you might consider pointing them in direction of a link, or a contact if they’re interested in the image. Such an offer certainly fits into the niceness angle I’ll be addressing…
Coming from another angle—don’t go into this with a holier-than-thou attitude. Nothing puts a person off quicker than a photographer with an attitude—a sort of art school mindset that everything must be just so, and that you are entitled to their picture simply by virtue of being in the same area. You wanted a photo of them specifically. It’s their right to give it to you or not. Don’t get snooty, and don’t be offended if they say no.
Be friendly. If you’re at a food stand, buy a hot dog or an apple or something. Engage them in conversation. If it’s a street performer, consider tossing in a few coins for the trouble, and the lovely performance. And disarm them with a smile!
Well, that last bit sounded tacky, but hey, smiling works. Try being a scowly photographer, I dare you.