I have to face the fact that summer is over. It's been nice, but with our typical rainy fall here, I can't avoid riding in the rain any longer. Do you have everything you need to keep riding all winter long? Besides a trainer? :-) This list is geared towards the northwest, so modify it if you climate calls for it.
They key to fall and winter riding is to keep warm and dry by dressing in layers and wearing adequate protection. A long-sleeve wicking base layer is key, whether under a short sleeve jersey, long sleeve jersey or jacket. Wool is great for this, since it retains heat even when wet. Having layers you can strip off as you warm up will also keep you comfortable. For example arm warmers and vests are things that are easy to take off and stuff in the back pocket of your jersey. Having a cheap, clear plastic rain jacket is excellent to start out with rolled up in the back jersey pocket in case you encounter rain. Also make sure you have enough food with you, as you'll burn more calories in the winter, trying to keep your body warm!
Here are some things that are essential, and some things that are extravagant!
Arm warmers - great to strip off if the sun comes out on a beautiful fall day!
Knee warmers - you really should have your knees covered if it's below 60 degrees out. Keep those joints and legs warm and happy!
Nylon vest (with mesh back for ventilation or solid)
Anti-fog wipes for glasses - great for keeping your glasses from fogging up when you stop. Can usually get them in ski shops.
Wind front tights
Warm Jacket - I would recommend something wind resistant, but not necessarily a nylon jacket, as they tend to be billowy.
Skull cap - has to be pretty cold to warrant this, I usually wear an earband.
Long fingered gloves (duh!)
Lobster gloves - these are like mittens, but keep two fingers together, like lobster claws. They are very warm, and are great for 30-40 degrees.
Gore-Tex Jacket (pit zips are important to prevent overheating)
Gore-Tex Pants (a little overkill, except in torrential rain!)
Plastic rain jacket - these are clear and available at all bike shops
Gore-Tex gloves - it's great if you can find these in a shell to wear over your regular gloves.
Gore-Tex oversocks - these go over socks, but inside shoes. Can be a little overkill on anything but torrential rains.
Bike hat with small bill - you've seen people on the street wear these, but they're great for keeping the rain out of your face.
Wool socks - keep you feet warm(er) and retain heat when wet.
Fenders over front and back wheel with a mud flap in back!
Puncture resistant tires
Flashing red light for back of bike
Light for front - if you're doing a lot of riding in the dark (esp. commuting), investing in a rechargeable battery headlight, like Night Rider or Light 'N Motion is key.
*** Be sure and take care of your bike when you're riding in the rain. Nothing wears the parts out faster than the grit on the road. If your bike gets wet on a ride, it's best to gently hose it off after the ride. Wipe off the chain as well, and remember it will need to be re-lubed after it gets wet. Always plan on lubing it after every 3 - 5 rides regardless.
Good luck! I'll also post a list of my favorite on-line shopping places later, to help you get ready.