Disclaimers: Everyone is different. Expected weather should play a huge part in your decisions. Ride distance and level of support matters. You should treat this as a list of ideas and not gospel. Your mileage may vary. And of course, any list like this will omit something through oversight that is absolutely essential.
Tricia and I are planning a multi-week bike trip in mountainous regions of California. I’ve been asked what sort of clothing I’ll take. I’ll have support for carrying my bags and tent, so I can be a little less streamlined with my packing. I have options not available to self-contained cyclists. This is my working list, from top to bottom.
On the bike clothing
Helmet, of course. Do I really need to add anything?
Buff. What a weird word for such a useful piece of head gear. It’s just a tube of fabric, but what a difference it can make. I use one on my early-morning bike commute and it keeps my ears warm without being as hot as a hat. I'll take a fleece hat too, but mostly just in case it gets really cold at night.
Base layer. I love Under Armor. I can’t figure out why it costs as much as it does, but after trying numerous knock offs I like it the most. I wish I could find it on sale. For me, their Heat Gear models are best as I’m not riding in below freezing weather. I find they fit me well and I love how they wick and how quickly they dry. Tricia likes wool a lot. She says it keeps her warm even if it’s wet. I don’t like how it feels on my skin, or how long it takes to dry. I’ll take short sleeve versions for flexibility.
Arm warmers and knee warmers. I like them because I’ll only bring short sleeve base layers and jerseys to wear with shorts. The warmers are great in the cool mornings and can be peeled off when (or if) the day gets warmer. I like DeFeet Kneekers for knee warmers. DeFeet arm warmers are pretty warm, sometimes too warm, I may bring them, and also go with some lighter-weight models.
Jerseys. I like Coolmax/magic-fiber jerseys with gaudy graphics and pockets in back. Like tee shirts, they should reflect your inner philosophy and dreams as well as help people pigeon-hole you quickly.
Jackets.We both have very light-weight jackets. It’s hard to see how lightweight they are in photos, but they fold up very very small and fit in a jersey pocket just fine. They're more like super-thin windbreakers. It's amazing how often just that little bit more is just right.
Gloves. I’ll bring regular half-finger gloves and some glove liners to wear under them on cold mornings. I’m a self-confessed hand wimp. I hate having cold hands. I ride with people who never seem to have this problem. Full finger gloves? Maybe.
Shorts/Bibs. I realize that people are bib folks or shorts folks. I won’t try to change your mind. I’m bring comfy ones that have tight enough legs to hold the knee warmers on so they don't slip and create unsightly knee-warmer gap.
Socks. I may not be a wool base layer fan, but I love wool socks. They keep my feet warm when the morning is cold and rarely seem to get too hot. I’ll also bring a few lycra socks for hot, hot days, and because lycra dries quickly. The fact Lycra/Coolmax often have cool designs I enjoy did not affect my decision making at all.
Shoes. I love, and my feet love, my Sidi’s with Shimano SPD-SL cleats. They’re the big ones that are a pain to walk in. A lot of people like mountain style cleats and pedals for their off-the-bike walking friendliness. If I had tons of cash I might try new pedals and shoes for this trip, but I’ve used by Sidi's on past trips and survived the walk-like-a-duck-on-ice issues, so I’ll try it again.
Toe covers/warmers. These little bits of neoprene can really make a difference if you have well-ventilated shoes.
Rain gear. I have a Showers Pass jacket and rain pants I bought to commute in. I’ve heard that you can try all the other brands, but you’ll eventually get a Showers Pass. Their stuff sure seems high quality.
Not clothing, but stuff to carry on the bike:
On-the-bike bags. I have an slightly oversized seat bag that I’ve used before, but it isn’t huge. I bought a Revelate Designs Pika Seat Bag, which is bigger than huge. I’ve tried it on local rides, but not on anything serious. I think it will be big enough to hold the usual tube, tire levers, and patch kit as well as lunch, rain clothes and anything I shed on the ride. It looks really silly, but I have high hopes it will work. Tricia has a handlebar bag. I just can’t do that due to a early exposure to very strict cyclist aesthetics that limits my world view. But Tricia likes it fine. I'll also use a Banjo Brothers "Bento Box" top tube bag to hold my camera. I hate how it looks, but at this point looking cool is just a distant memory.
Water part 1: Polar water bottles provide at least a little insulation.
Water part 2. Because in the middle of nowhere it can be many miles between water stops, in addition to two large bottles each we carried a "Platypus" water bottle/bag. They hold half a liter and fold up very small when empty. I stick one in my pocket and Tricia keeps one in her handlebar bag when we have concerns about water.
Rear light. After riding with friends in Portland we realized that going in and out of shadows can make a cyclist really hard to see. We bought Cygolite rear “blinky” lights. They’re USB rechargeable. I’m amazed how bright they are even in the day time.
Pump. Pumps are for people who need to stop and help CO2 users when they inevitably screw up their CO2 inflaters. Tricia and I both like small pumps with foot pegs. For those of us without massive upper body strength the foot peg is totally rocks, making a small pump more like a floor pump. Topeak and Lezyne both make nice models.
Emergency tire-problem solver. We are fans of Park Tools tire boots as well. Read our tale about them. I think you’d be crazy to leave on any long ride with a couple stashed in your saddle bag.
Bonus thing. I got a Sea To Summit Ultra-Sil Daypack that is super small, even tiny, when not in use, but is expands to big enough to haul some swag from a bike shop or roadside fruit stand.
Cool, but entirely unnecessary. I love having a Garmin on my handlebars. It doesn’t mean much to me when I’m riding, but having maps and elevation information of where I rode is really fun. And, being a geek, I also like having heart rate and power data. It doesn’t make the ride better, but it makes remembering the ride better. I use a Garmin 520.
Power. (Not carried on the bike.) Having an iPhone, Garmin and rechargeable rear blinky means lots of charging. I have two items to help me with that. One is a 5 port USB charger so I only need to use one AC outlet to charge all my junk. The other is a rechargeable power “brick” that has a USB port that claims to be able to recharge a phone 2 1/2 times before it needs recharging.
Extra bonus comments on stuff. I hadn’t thought I’d go through off-the-bike items. You know you need a toothbrush already, right? But here's one item we’ve discovered that is really good for us, and is worth mentioning.
For our last trip we bought Thermarest Neoair mattresses. I hated spending the money, but they are really small — less than half the size of our last pads — and very comfortable. I'd heard they were noisy to sleep on, but I didn't find it bad at all. I read the newer models were less loud, so maybe that's why. Space saving and weight saving, with good insulation and comfort makes for a good night's sleep.
Is that enough? It seems like a lot, but sometimes you just can’t buy what you’d like on the road.
I’m not even going to try and write about what you should take for when you’re off the bike.