How do you take photos of strangers without them knowing?

Get a good telephoto lens, or get professional to help you better deal with the fetish.

This is a good discussion. There are certainly some ethical considerations - the 'zoo' feeling, taking an image against people's will - that are balanced against the chance to get a 'good' photo.

Another consideration no one's raised yet is that if you ever hope to publish your photos in any kind of non-amateur setting, you need a photo release, which means you need people's permission to use their image.

Like Mr Trips, I've used any number of techniques including this one he doesn't explicitly mention - take the picture you want. Then, introduce yourself and ask to be able to take a shot. Often, the more posed ones are inferior for a number of reasons, but you've still got that first candid one and the subject's permission.

on the nonamateur setting... If you are selling as stock for commercials, you do then need permission. If you are selling your photo as 'art' or 'newsworth' you do NOT have to get permission to use the image (in the US at least). see Nussenzweig v. DiCorcia

Be respectful and try to be mindful of what you are trying to capture.

I'm even bold to preset as much as i can, put the camera to your face, frame, focus and fire -- most people don't know what hit them.. =)

On the flip side, I also sit down an talk to people before photoing them.

On both sides, I use up too much film.

There arer two ways for me nowadays:
- I ask people at the market if I can photograph their stuff displayed. Going very close and using superwide angle they are in the frame as well without knowing.
- I position myself with a zoom, usually 80-200mm, in a place in the mid of a crowd. I focus on one spot and snap away if someone walks into my 'field of fire'. People usually don't mind that as they walk into my way, I do not intentionally shoot them. A big grin from my side usually after the snap. Other folks around often laugh too...

- But most likely I get involved with the people, spend a long time, sign language, smiles, take some shots much much later.

Trying to steal a shot, then turning away as if it never happened is the worst in my opinion as if people do realize it then the really must feel like being in a zoom. All the tricks to steal a shot are quite embarrasing if you get caught - and sure enough you will get caught. More often than not.
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Oh, and I like to use small lenses. Albeit I use the huge 80-200 2.8 sometimes, I usually prefer the 85mm another fix lens as the smaller lens scares off people less.
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I just shoot the backs of their heads.