Just talk to people.
That doesn't mean you have to ask everybody for permission. Just simply chat with people.
Sit on the park bench and talk about baseball or something. Be polite. Be social and engaging. When people are comfortable with you, it isn't uncommon for them to come up and ask for a picture. Once or twice a week, I'll have people come up to me, hand me their digicams and ask me to take their picture. I always oblige. Then, before the scene breaks up, I quickly snap one with my camera, hand them my business card. I tell them to send me an e-mail and I'll send them a picture. Once the ice is broken, I can take almost any picture I want so long as I'm not an a$$hole about it.
The effect sometimes snowballs. When other people see you talking to other people and taking their picture they'll often come up and ask for their picture taken.
If you get yourself a box of Ilford postcard stock, you can mail the pictures you take to the people who want them.
Use stick-on return address labels with your business logo on them. Postcards make good advertising.
(If people get sketchy about giving out their home address, tell them it's okay to use a business address.)
Bottom line: Don't be that "creepy guy." Be a nice, sociable and professional photographer. People will treat you accordingly.