Bicycling is really hard to dress for because you can be laboring up hill at eight miles per hour, then not working at all downhill and going 25. So you go from overheated to chilled—just like that. Layering is the key. I use about ten basic pieces:
1. Patagonia Capilene 1 Silkweight short and long-sleeve Ts. These are ideal year-around base layers, or combine under a light jacket in cool weather. The short sleeve is '39, long is '45.
2. Icebreaker Bodyfit 200 Contour Crewe ('60) and long-sleeve Oasis Crewe ('70). These are Merino wool pieces that are the PERFECT cold-weather layering shirts.
3. Izumi Infinity Jacket and Zephrr Vest ('70 and '60). Perfect windbreaker-type jacket. Very light. Somewhat water-resistant. The jacket is the most useful piece I have.
4. Performance's Radiator Tight with chamois ('70), or Nashbar's Mansfield Padded Tight ('70)
5. REI's Novara Verita ('190), which uses eVent fabric—fantastic stuff. Or look at the Showers Pass Double Century ('125) which uses a proprietary waterproof-breathable laminate
6. Seirus Softshell Lite gloves ('45) are fine to 40 degrees. Colder than that, Pearl Izumi Barrier Gloves ('70).
7. Any synthetic briefs are fine. Ex Officio Give-and-Go Boxer Briefs are '25.
8. Get some sort of cycling beanie for temps below 45.
9. Get some wool cycling socks (Smartwool PhD Cycling Light Mini Socks, '17). And shoe covers (Descente Element, '40).
10. Arm warmers and knickers are nice for those in-between days, but optional.
Here's what it looks like:
From the upper 40s to mid 50s, I'll wear shorts, maybe knee warmers or knickers, a Patagonia silkweight T, bike jersey, vest, arm-warmers. If it felt chilly, I'd wear a wool T instead.
Lower 40s: Knickers or midweight tights. Silkweight T. Long-sleeve wool T. Zephrr jacket.
Upper 30s. Tights, wool short sleeve and long sleeve Ts, Zephrr jacket or rain jacket, shoe covers.
A lot depends on your temperature preference. You'll get the hang of it. I've found I can pretty much dress in 2 to 3 degree increments. I've even biked when it was 12 F a few times and still looked like a cyclist–not a skier. But that's getting pretty cold, especially when you generate wind chill of -10 or so on the downhills.