When I'm on the street I usually just walk along, and only ask people when the situation presents itself. Where I live,it's a predominantly pedestrian town, so a lot of times you can catch people at crosswalks, or when they're just waiting around. Try not to take much notice in them, and above all, try not to act interested until you get right next to them.
It's funny, because I have to use this same strategy for spearfishing. If you intentionally look at fish right before you spear them they'll swim away 9 times out of 10. I don't know if people have the same "sixth sense", but fish sure do. Try to be as nonchalant as possible, almost as if you could get anyone to let you take their picture, and you're giving them the privilege of getting their picture taken.