Very few cycling bags are completely waterproof, so we suggest that you pack your personal effects in plastic bags. Use several small bags rather than one large one: this saves space and is more practical.
Bring dry clothing (T-shirts, socks, polar-fleece sweater) to change for dinner. You’ll be glad you did!
Bring a two-piece set of good raingear (jacket and pants). It should be light and have air holes. Gore-Tex products are excellent. This fabric is breathable and provides protection from the rain, but it is quite expensive. Avoid ponchos and firefighter coats. Ponchos create too much wind resistance, and firefighter coats make you sweat so much that you will get just as wet as you would without raingear.
To keep your feet dry, you can buy special overshoes at bike shops. Another solution is to wear woollen (or polar-fleece) socks inside your shoes. Your feet will get wet but will at least stay warm. Avoid cotton socks and makeshift solutions such as wrapping your feet in grocery bags, because your feet will tend to slip off your pedals.
Lastly, don’t wait for a downpour before putting on your raingear. It takes just a few minutes to avoid being wet and cold all day long.