I went into a local shop to buy some tubes or something a couple of years ago and the owner said "Hey, do you know how to install a headset? I'm without a good mechanic right now and the board is filling up." I started right then.

The shop ended up being a pretty weird place to work (lots of family drama, Christian overtones, etc) but it was a great learning experience - regarding both what to do and what not to do - when it came time to open my own shop.