[PilotPirx]

You can get a lot of good clothing in bike shops that you can wear over your normal clothes (shoes, trousers). Very good gloves, since your hands will freeze first.

A backpack to store the clothes when the weather changes. (best if your laptop can fit into it if you need to take it with you.) I have a backpack with a kind of raincoat, that can be pulled over it. Saved my laptop several times. Take a few plastic bags with you, when it rains heavily you can protect your equipment.)

good lighting equipment and reflectors are important if you bike in rain or darkness.

[malandrew]

If you have papers or a laptop that can't get wet, get a nice messenger bag made of Cordura and truck liner. I highly recommend PAC Bags, which are easily the best made bag I've ever used.

http://www.pacdesigns.com/