The number one rule, I beleve, is don't be sneaky. You are using people for your own ends, so they deserve to know what you are up to. My exeperience working in urban photography has been that people are mostly nice when you talk to them. Ask permission when it seems appropriate. If you are just shooting the passing scene people don't care.
Eye contact makes all the difference in a good image.
Zinn A . A question I have of other street photographers is what you do personally about model releases. I have exhibited select works in group shows, but I have plans on a show of my own work. Though I know I should contact an attorney to find out exactly what my legal obligation is, I am curious as to what other street photographers who exhibit or publish their work do. I know where I stand in regards to commmercial use without a model release, but I am unclear about it when it comes to exhibitions and monographs.