A light coat of oil as you normally would when you clean your gun will go along way to protecting it from the effects of rain.

If you do get soaked, make sure that when you get home that you allow your gun time to reach room temp before cleaning. Never put it back into the cabinet without cleaning it properly.

If you fail to let it reach ambient temp, especially if it was a cold day, the metal will sweat, even if you already wiped it down, which can cause moisture to form... leading to RUST! Personally, I wipe all my guns down, even if I just handled them for whatever reason. Your fingerprints alone can also cause damage to your guns finish.