For an experienced runner, you may not find too much help in this because you probably already know what to bring and wear. But for first timers or those newer to the sport, maybe some of this will help.
If your wondering if I know what I am talking about. I will tell you, I have run 26.2 in snow and freezing cold weather. Soaking wet freezing ice and hail the entire 26.2. Warm and wet off and on races in several races. Hot marathons in the upper 90's to 100. Windy races that the wind just cuts right through you and freezes you to the bone no matter what you do... Of course I have also ran quite a few perfect condition races too:) I've survived them all quite well based on my past experiences!
However, as always the post below is purely my opinion, and should only be interpreted that way.
COLD & DRY WEATHER
Most distance runners prefer colder temps, personally I think 40-52 degree's are the perfect running conditions. However, sometimes those temps can drop into the 30's or below. Here are my suggestions and what I wear for those times.
Gloves - Cheap throw away ones.
Ear Covers - A cheap throw away one, or a larger headband that can be moved around works great.
Pants & Long Sleeve - Just for the start, use cheap stuff you can throw away. However if the race looks to be going to be cold the entire time, you may want to invest in pants or long sleeve shirts that are sweat resistant and made to run the entire race in.
Garbage Bag - You don't need the huge ones that go to your ankles, but one that hits your knee's is perfect. You can sit inside the bag and keep warm before the race (it will keep the breeze out) and you can run the first mile or so inside the bag and throw it to the side of the road when you start warming up and sweating.
Hat - Optional, but I almost always go with a light weight moisture wicking cap just to keep a little of the heat in.
COLD & WINDY & DRY WEATHER
Windbreaker - If it is a cold wind, this is an item to bring, especially if you think you may need it the entire race. Preferably one that is a little on the breathable sweat proof side, however if you start to sweat in it, you'll want to take it off or unzip it until you cool off again.
Gloves - Preferably a pair you can toss, or keep in your belt or waist band until you need them or to use off and on in the race.
Pants - If it is going to be really, really cold and windy you'll want a windbreaker type pant. But if you think it will warm up as the sunrises, I wouldn't wear a pant.
Garbage Bag - To keep warm in before the race, and to use in the first mile or two as you warm up. This can also be used as a cheap throwaway windbreaker. But toss it or take it off as soon as you start to sweat.
Chapstick - You'll want this for your lips, nose and possibly cheeks to keep windburn down.
Bandanna or Buff - If it is going to be really cold, in temps under 30, you will want to wear something over your nose and mouth to keep the cold out. But you also want this to be easy to remove and put back without much effort at all. If used correctly, this is how you prevent "freezer burn lungs" which let me tell you, is something you don't want.
Hat - I like a hat, so I can tip the bill down when I am running into the wind and give my face a break from the wind. Always a moisture wicking running hat/cap is best!
NOTE: If possible, if the wind is coming at you head on. Get behind someone and let them block it for you. They may not like it too much, so pick a few different people along the way. If you find you have others doing it to you, and it gets annoying. Slow down or pick the pace up and they'll move on to someone else (hopefully).
WARM(ish) & WET
When it's going to rain, less is more. It's also more important to wear moisture wicking clothing. If it's going to be warm or not too cold. Stick with your t-shirt or tank and shorts.
Moisture Wicking Socks - This is the most important item, as if you wear the right socks you won't have to worry about blisters.
Garbage Bag - You may or may not need it, but I always bring one just in case. It will easily fold up small into your belt or waist area and if you don't need it toss it, but if you do you'll be glad you had it to keep you warmer during the early hours of the race.
COLD & WET
Nothing is harder than cold and wet races especially if your running longer distances. You can start out fine and then end up freezing or getting beginning stage hypothermia toward the end. A few simple things can help prevent too much misery.
Keep it light as possible - Depending on the temps for that day, tighter fitting moisture wicking material is always best, whether that be t-shirt, long sleeve shirt, Capri's or pants. Make sure they are moisture wicking and material that dries quickly. Anything too bulky will just make you more wet, and weigh you down as the rain falls.
Gloves - Cheap throw away ones, but if its a race that looks to be cold and rainy throughout. I suggest taking them off as you warm up, and keeping them in your waist just in case you need them later.
Garbage Bag - For races in these conditions, I recommend bringing 2. One for before the race to use while your waiting at the start and in the first miles of the race if needed to help you warm up. Also one folded up in your belt to use later on if you need it. Later in a long race especially around miles 18-26 your body really starts getting colder just because of the energy spent. If you have a bag to put on if it starts raining and freezing you in those last miles you will feel it is the best lifesaver.
Waterproof Jacket - I actually don't like to run in these, they are bulky, and you get way too hot in them. But if it's a rainy starting line you may want one just for before the race starts. However, I prefer the garbage bag to this, it's cheaper and easier to handle and discard.
Hat - I like a light weight moisture wicking hat for races in these conditions.
NOTE: While running avoid puddles and running behind someone who is kicking up too much water, when possible.
COLD & SNOWY
Layers - This is one of those times, where you actually want a little more on you. I prefer windbreaker type pants (moisture wicking) and layers, you might even want a spandex type pant on under your other pant. Sometimes even 2 long sleeve shirts may be needed, so you can take them off or put them on as needed throughout the race. I don't toss these items, if it is going to be cold and snowy, you'll likely end up running the entire race in your different layer options.
Gloves - Cheap ones, and I prefer those cotton mitten ones. Your fingers stay so much more warm when together, then separated into different fingers. Don't throw them to the side if you get warm. If the weather is going to be snowy throughout and cold. Keep them in your waist if you don't need them for a short time, because you may need them later.
Socks - Moisture wicking. I never run in heavy thick socks. But they do make a slightly thicker moisture wicking sock for runners. However, never run in anything you haven't tested first.
Ear Covers - You may want a thicker material or item for this, but make it something you can take off and put on easily if needed.
Bandanna or Buff - To keep your mouth and nose covered to keep the air you breathe in warmer. Something that you can easily push up, or pull down as needed during the race.
Garbage Bag - Yup, one for before the race and for the first few miles. Again take it off as soon as you get warm. However, bring a 2nd one just in case things get cold in the later miles. Then you can put it on and run a mile or two in it and warm up again. Just remember take it off when you start to sweat.
Chapstick - Lips and nose, maybe even your forehead and cheeks. Water drips right off and the cold won't see quite as bad.
Hat - Moisture wicking is best, but even if you go with a cotton one, you'll appreciate the warmth your body will keep and not loose through your head.
You probably noticed I listed Garbage Bag a lot!
Well this is the cheapest and lightest weight thing to keep you warm and dry.
The key to using the garbage bag correctly.
Start with just a hole poked in the top for your head. Keep your arms inside the bag.
As you start warming up, poke your hands through the sides of the bag.
Once you start sweating and warming up, take that bag off!!!! Don't keep it on just because its cold out, if your sweating, take it off! If you keep it on and sweat, you'll just make yourself colder. The same goes for any warm up clothes (pants, long sleeve shirts, jackets) if you keep them on as you start sweating, you're just going to make yourself colder as they get wetter from your sweat!
I always bring at least 2 garbage bags with me, especially if there is chance of rain and cold. One for before the race, and another folded up tiny to pull out in case of emergency later in the race.
Usually, you'll only spend a mile or two in the bag, but I once had a very rare occasion where I was running in temps around 30 degree's in high winds and it was pouring rain. Freezing pouring rain the entire race, and later on in the race it started to hail and that wind just whipped that rain and hail right into me. I ended up spending all but 2 miles in the garbage bag.
I kept pushing it up, pulling it down. Keeping my hands in or out as needed. I was one of the few runners who didn't have major hypothermia problems, and even though they closed the race down shortly after I crossed the finish line. I guarantee I would have been further back and not allowed to finish if it wasn't for that amazing black garbage bag! (If your wondering, I kept the bag, I had to spend 26.2 in that bag, it was going in the scrapbook!)
I could go into more detail and add more stuff, but that is what I use. I am always a Less is More girl, and cheaper on some things. But when it comes to shoes, socks, and the main clothes I am running in, that is the time where You Get What You Pay For. I spare no expense in the things that are going to keep me comfortable!