It's not fun, but if the heavens open as you're leaving for work / home what are you gonna do?
Kit - well I've got some Rev'it Axis waterproof pants that are a size too big, but slip over my work clothes nicely. My Dri-rider jacket has got removable waterproof and thermal inner linings and so far so good! Boots can cost a fortune so I opted for some Dr Marten boots, which I can get away with wearing at work too.
Riding - I think the best advice I could offer is ride confidentially and take it a bit easier than normal, but don't ride like a grandma as you'll probably get into trouble being overly cautious. Ride at 80% of your usual pace and avoid trundling along in higher gears at low revs. Lower gears with higher revs can be easier to control as rolling off the throttle will slow you down in addition to breaking. When cornering consider slightly increasing your distance from the vehicle in front and go in slow, breaking when cornering in the wet is not fun and you can always open up as your coming out of the bend.