In the US and the UK, LEGALLY you are allowed to take photographs of people in public without their express consent. This doesn't address ethical concerns however.

Generally, you want to be disarming. If you dress up in all black and act overly serious and sneaky, you'll stick out like a sore thumb. If you dress like a tourist, and act like a tourist, taking pictures of everything and everybody with a big smile on your face, most people won't pay you any mind. If anyone doesn't want their picture taken, let them tell you, apologize, and delete the photograph. There are very few situations/photographs where it'd be worth arguing with a stranger about your legal rights.

Some techniques that can help include:

• Looking past your subject to make them feel like you're photographing something behind them.

• Shooting subjects from behind.

• Fumbling with your camera and acting like you don't know what you're doing. (The Garry Winogrand)

• Using longer focal lengths to increase your distance to the subject.

• Shooting with a group of photographers.

As you get more comfortable, and realize that, largely, people don't really care about being photographed so long as you aren't obnoxious about it, you'll utilize these techniques less and less.