i have done this .. not recently but i have done this, and often ..
if you are on the street .. have a camera set up
i see from your profile you use a 4x5 or lager camera use that ...
maybe ask some friends you can "fake it with" so the people know what is actually going on ..
talk to them, engage them in conversation, tell them you are a traditional photographer
and you want to take their portrait with whatever contraption you have. give them a business card
and something YOU sign saying you won't post their photograph on ANY website without their consent
don't dress like anything but "normal" ( whatever that means )... get their name and email ( or give them a time and place
you will meet with them ) and when you have the photograph made show it to them, and GIVE them a copy and ask their permission to
publish or have a show with it, or have it in your portfolio.

over the years i have photographed a lot of strangers, on the street or in some sort of work environment. never really had a problem
except for once or twice ether when the person was trashed or totally paranoid ...
after a while i was hired by a company to do something similar. but they made the contacts / and i arranged to photograph the strangers.
it was still tense and sometimes strange .. and it usually worked out ..

expect a lot of rejection at first, working on the street making portraits like you plan to do, is very hard, leaps and bounds harder than
doing stealthy street photography with a long lens and 100 feet between you and your unknowing subject.

good luck !

john

ps. some brain feed
http://roarkjohnson.blogs.com/photos/stranger_a_day/