I've never done it but usually when someone blows my mind with a wicked drum solo or something, I'll throw em a few bucks. Anything that's generally totally kick-ass, I'll throw some dollars. But if it's plain and boring, I just walk by. Be innovative, or really passionate. Sometimes for me at least, I'm more entertained by just crazy techniques and stuff, doesn't have to be a whole song. But if you're gonna be doing it for a while, you prolly wanna throw in some full songs. I dont know, just my thoughts...
No experience, but what I would do is jam 'till you get a small crowd, then play a full song, assuming you can sing.
I think Natalia Paruz says it best... she's a world renowned sawyer, and she has a page dedicated to busking, her experiences, and what it means. I think she pretty much says everything worth saying. http://www.sawlady.com/sawmemo.htm

She also has the coolest story about busking I've ever heard -- it's the one about the plant.
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