Originally Posted by MichaelW

Look for a waterproof jacket with pit-zips, a back vent and a front zipper with poppers. Modern lightweight WPs have a WP zipper with no poppers, it is either open wide or totally closed. Poppers let you ventilate the front on a wet climb. Poppers are much better than velcro, which can stick to a wool baselayer and cause pilling.
If you get to the top of a really big climb in cool, rainy weather, change your baselayer to a dry one, put your long tights and waterproof pants on, and add a midlayer. You will chill really quickly and you still have a long, cold descent to do.
It is more difficult to dress for rolling country, a few minutes of climbing, a minute of descending, repeat x20. Use your gears and limit your work-rate on the climbs.
Waterproof socks such as Sealskinz work best in conjunction with waterproof pants, otherwise water drins inside the tops. After a couple of days of wet cycling it is your feet that really suffer. Keep a supply of dry socks.