Packing the right clothing is tricky. Unlike a short bike ride, we're touring. Which typically means on and off our bikes throughout the day, whether to visit a cafe, tour a site, restock at a grocery store, or hike a national park. It can also mean stopping a couple of dozen times during the day's ride to check directions. And the more days in the tour, the more likely you'll experience some type of inclement weather.
I want clothes comfortable on and off the bike, while still being ready for wind, rain, or worse.
No matter the time of year, I have a short list of base clothing. For shoes, I prefer waterproof, low-cut hikers, rather than my dedicated cycling shoes. On an unfamiliar route it's key that I can drop my foot to the ground in an instance - don't want my feet clipped into my pedals. Considering that I'll be walking about in public, I take two pairs of baggy bicycle shorts, with a padded lining that can be removed and washed as needed. Cotton or synthetic shorts, shirt, socks, and underwear for days off the bike, or at the campsite. Merino wool is great (mentioned below), but it's pricey. Cotton is cheap and easily replaced.
As soon as I hit camp (or hotel), it's a shower and a change into "evening" wear; sandals and lightweight athletic/jogger/soccer style polyester pants. Good for cool evenings, or any occasion where I want long pants.
This is the easy one. I've heard cyclists pride themselves on taking one shirt and pair of socks for multi-week tours. With the caveat that no one wants to be in a crowded elevator with them. I prefer two of each. Look for lightweight Merino wool; it provides insulation, wicks away moisture, and doesn't stink up as much as other materials.
Switch out short sleeve shirts for long-sleeved ones, and lightweight Merino wool for something heavier. Layering is key, so I carry more changes of clothes. Plus if it's cold and rainy, it's important to have something dry to change into. I only use a rain jacket for cool/cold weather; no matter how "breathable" manufacturers claim their material is, I still sweat too much in warm weather. The same goes with waterproof gloves and pants - it has to be pretty cold for me to put these on. And if you have a fairly large handlebar bag, you'll find that it'll provide decent protection for your hands from wind and rain.
Fleece is a favorite for a lightweight jacket. Fairly inexpensive and warm - I pick a lightweight version and wear it under my rain jacket on those really cold, wet, weather days.
For a first-hand account of touring clothing, Bicycle Touring Pro lists his Bicycle Tour Clothing Essentials
Check out the first section in Adventure Touring Cycling Association's Touring Gear Essentials for a rundown of clothes for on and off the bike. Don't skip the rest of the article - it's good as well.
A structured approach to the all-important method of layering is covered (get it?) in BikePacking.com's ESSENTIAL LAYERING FOR BIKEPACKING: THE SIX-PACK METHOD