First off, since no one else has said this -- make sure your string is well waxed. Water can wreak havoc on strings.

A local bow shop told me a neat trick to wax a string -- apply wax and then use a low heat on a hair dryer. It will soften the wax up a little and soak into the string. Do this a couple of times and you'll have wax all through the string.

For the rest of the bow, a light oil seems to work best. WD40 is not a lubricant but a solvent. It will just strip all the oil / grease / lubricant that is on the bow away. (Can also lead to more rust). It has it's place, just not to protect from water IMHO.

Good luck! Hunting in the rain can be fun if you can manage to stay dry!