I can only give you one bit of advice, which is that if you ever consider using your images you take on the streets for commercial use, you should have a good law firm inform you on the different situations and uses of photos. On one side of the spectrum you have photojournalists, who have the (sort of) universal right of news gathering - so they never to ask permission (just imagine they'd have to!) or risk getting sued all the time for this stuff. And on the other side is using someone's photo or portrait for commercial use, like using a famous person's face for a add campaign. I guess with 'amateur' or hobby photography it's very up to the situation or kindness of people. Although personally I try to avoid staring at the people that I shoot, to give them the idea that I shot the person next to them or something like that ;) Usually works...just be quick =)