First find your victim, sorry, I mean subject, point your camera above their head as though you are taking a pic of buildings etc keeping one eye on the subject, as soon as they look away, or look at what your pointing at, drop the lens down and take the shot! works for me!
Looks more natural and un-posed. Unless you dont want it that way of course.
And if you follow this advice, you'll get a bunch of random shots of the backs of people's heads.
OP:
Approaching people on the street is simple. The main thing you need to constantly keep on your head is that rejections don't matter. There will be people who don't mind being shot at all, people who mind, but can be persuaded, and people who do mind, can't be persuaded, and react to you like you're a leper.
Approach people with a smile (not too big), and be politely assertive. Try not to
ask as much as
tell them you're going to take their picture.
Just something like: "Hey, I'm doing a project (never say "personal", they'll just hear "creepy"), I wanna take your picture, just over here-->".
Make sure they're not actually standing in the place you want to shoot them, but catch them a few feet away and then point and say "You just have to stand right there". Flatter them if they look nonplussed and tell them they're perfect. Perhaps explain the project to them if it's a good one.
Telling them where to stand gives them their direction, engages their brain, and in most instances puts them under your control.
Take a couple of shots with natural light, show them the back of the camera, and then say "I'm just going to try one with the flash as well", then attach the flash, take the shot, and hand them a business card. (Don't be tempted to hand the business card over first, as you'll risk looking like you're pitching for business).
That's how I've always done it if I need people for a composite.
Just remember that a lot of people will refuse, and it's got nothing to do with your approach. Some people jus don't want to be photographed, and there's nothing you can do in the 5 second window they will unwillingly give you before they walk away with their excuses. Factoring in rejections, you'll average about 25 portraits per hour if you are in a busy area. If you're in a busy area with young people, you'll average about 20,000,000.