Dress in Layers if It's Cold
If it's very cold and rainy, you may need to wear a couple of layers. The most important layer is the one closest to your body. Make sure it's a technical fabric such as polypropylene or CoolMax, which wick water and sweat away from your skin. Your outer layer should be a wind- and water-resistant jacket or vest. Don't wear a waterproof rain slicker because it will trap moisture and heat.
Wear a Hat With a Brim
A hat with a brim can be your best friend during a rainy run. It will keep the rain off your face, so you can see, even in a downpour.
Don't OverdressThis is one the biggest mistakes runners make when heading out for a rainy run. Wearing more layers will not keep you dry. Unless you're running with an umbrella over your head, you will definitely get wet. If you have tons of layers on, you will just be wearing more wet, heavy clothes. Dress for the temperature, as if it were a dry day.
Be VisibleSelect an outer layer that's light-colored or has reflective strips, since running in the rain often means poor visibility.
More: Top Visibility Gear
Use a Garbage BagIf you have to wait outside in the rain before the start of a race, a big trash bag with armholes and a neck hole cut out can help you stay dry. You can take it off and throw it to the side once you get moving.
Wear Old Running Shoes at the StartIf you're running a race, keep your race shoes and socks in a plastic bag while waiting at the start. You can check your old shoes in your gear check bag, and put on your race shoes and socks right before you head to the starting line. If it has stopped raining by then, you'll be able to run the race in dry shoes and socks.
Prevent ChafingIf you're running long, spread Body Glide or Vaseline on parts of your body where you would normally chafe or get blisters -- such as your feet, inner thighs, underarms, sports bra lines (women), and nipples (men).
More: How to Prevent Chafing
Protect Your Electronics
Store electronics, such as your cell phone and iPod, in a ziplock bag or a water-proof carrier.
Just Run!The hardest part of running in the rain is often just getting started. Once you begin running and warm up, you may find that you actually enjoy it! And it's good preparation in case you ever have to run a race in the rain.
Dry Out Your ShoesWhen you get back from a wet run, take off your running shoes and stuff them with crumpled balls of newspaper. This helps the shoes keep their shape, and the paper draws moisture away from the shoes.
More: How to Take Care of Your Running Shoes