Get wet! It’s actually pretty fun to run in the rain. Some pieces of gear are necessary (or at least useful) during a downpour. Wearing a hat will keep rain out of your eyes. Using a raincover for your pack will keep everything dry. Some wet weather clothing helps, such as a goretex running vest or jacket, but in certain conditions they can make you overheat, so pay attention to your body and make adjustments as needed. Drymax socks will wick sweat and rainwater away from your feet.
Dress warmly. Find a good hat and a pair of gloves in the fall, and make sure they keep you warm enough (fleece works pretty well.) Protect your face with a balaclava or mask if the temperature or wind chill goes down below 20 degrees (F) or -7 degrees (C). If you have a long commute, you might want to carry a handheld. Remember, you still lose water when you run in the cold!
Apply sunscreen before you head out. Dress in loose-fitting, breathable clothing. Carry one (or two) handheld water bottles. Find a place or two along your route where you can stop and fill up on water. Don’t push yourself too hard.