Sealskinz socks or Wellington boots (not for the clip-less crew, I guess), rain jacket and pants, sealskinz all-weather gloves... Dry bag and change of clothes - condensation and sweat 'll get you in the end.

If I get too warm I roll my sleeves and pant legs up, but this means the waterproof socks and gloves quickly get soaked inside by water running down legs and arms. Any solutions to that out there? I've been wondering if wearing tennis sweat bands just above the gloves and socks would help somewhat, but haven't experimented yet....

If it's warm I'll just smile and sing in the rain