Some people think that it is impossible to make a living at stock photography unless you have a studio. Images of people seem to be the big sellers, but, from what I have seen and in my very lacking experience, I don't think the images shot in a studio are necessarily the BEST sellers.
2. Once you have asked, make sure you are prepared to tell them what you plan to do with the pictures. If you are like me and you carry Model Releases in your pocket or car, then it isn't too hard to print out the "What is RF Stock?" page from the FAQ page and let them read that. It looks professional, they can even keep it and look the site up and if they decide, after viewing the website, that they don't want you to use their pictures, then they can just call you (hopefully you were smart enough to give them your card or scribble your first name and the 7 digits on some kind of paper-like substance). They may not ask about this right away, and it may not be what you want to bring up right away, but you can work it into the conversation after you have taken a few shots.
3. Make sure you keep it light and be confident. Most people will be fine with you taking a few pictures of them, signing a model release, and letting you email them the link to their picture online so they can keep track of how many times it was downloaded. It is as fun for them as it is for you. But, if you are a nervous wreck, checking every setting on your camera and apologizing every 10 seconds, then the whole experience becomes uncomfortable. If you shoot a few pictures of some neutral subjects in the same lighting before approaching someone, you will already be in the ball park.
Honestly, getting "THE shot" is not what you are going for. Getting a few "decent shots" will be alot more fun and less pressure. If you keep your camera on drive, you will be able to shoot away and maximize the few moments you may have with the person or people.
I tend to joke a lot and try to get them to forget about the camera. But, there is no real step-by-step for every situation. I would stick solidly to the first step and then see where it goes from there. Learn to speak their language.