I've far from mastered this, but a few thoughts..
Obviously good waterproofs will keep out the bulk of the water, but arriving bone dry after enduring a prolonged downpour is unrealistic.
Goretex shoes are great; on the one occasion my waterproof trousers channeled water directly into them and soaked my socks / the inside of the shoes, I swapped to a spare pair of socks while the other dried, swapped again and so-on until the shoes and socks were dry.
Light synthetic fabrics dry quickly, Merino remains warm while wet and takes a long time time to become aromatic. My new base layer has done maybe 120 miles of commuting without a wash, and only now vaguely smells like it needs one.
On account of the above you don't want to be wearing anything cotton.. I usually wear my riding gear all day but do carry a complete spare change of clothes incase I sh*t myself or have to impress a girl with my taste in polonecks..