dont' bother with the whole waterproof over-trousers or all in one bodysuit type garb. I've got an 'oxford bone dry' all in one waterproof suit but the one time i wore it I bonked 'cos it was so bloody difficult to cycle in.
The only thing is to stay warm, really. I don't personally mind getting wet as long as i'm not cold. The only thing that's slightly bad is when your stuff gets wet on the way in, and you have to put that stuff back on for coming home and it's still wet when you put it on again.
My personal rules:
a) never DONT' cycle to work simply because it MIGHT rain. (although I consider it acceptable to drive if it actually IS raining.) If it rains on the way in, either put up with it being slightly damp coming home again (ok now 'cos it's summer), have a spare change of cycling gear kept at work (what i'll probably do when it gets more wintery), or make use of drying facilities (if you've got them count yourself lucky!)
(
always cycle to work unless there's a specific reason not to. for me, that's usually something to do in the early evening like playing squash, or having to carry something like a parcel - but it's rare for there not to be at least two such instances a week, but i still normally get at least two cyclings in per week..