Some would say that getting hot and sweaty is the whole point of a good ride, but sometimes showing up either in geeky spandex or soaked and smelly is kind of unfeminine. So for those ocassions when something more practical than a sweaty workout is required, here's some tips for keeping the temperature in balance.
Watch the Weather
Weather is probably the biggest factor in how sweaty you get. On a dry, hot day, your sweat will evaporate so quickly that it will be hard to see it on your skin at all.
A hot, humid day, on the other hand, will leave you dripping just walking to the bus stop. The sweat evaporate more slowly in the humidity, and as a result it is less effective in cooling your body. Your body sweats even more to compensate, and as a result you get doubly soaked. Even moderate temperatures, when it's humid, can get a rider sweating enough to demand a greater water intake than usual.
The amount of heat that has to be dissipated, other factors aside, is directly related to the amount of effort being expended. While a heart rate monitor, properly used, is a great investment for building fitness, you can get by just by listening to your body.
When riding lightly, up to about 70% of your aerobic maximum, you could converse with a riding buddy. Pedalling will still seem easy at this point. As energy output rises, pedalling seems feels like light or steady effort, rather than of effortless or easy pedalling like walking down the hall. Talking becomes difficult at this point.
If you are trying avoid working up a sweat, on a dry or cool day you should limit yourself to this sort of effort. On a humid day, or if you are in the wrong clothing, riding any harder than you could chat comfortably is a bad idea.
Watch your effort on hills, and shift to an easier gear in advance. As the hill steepens and your effort level rises above a light, steady effort, breathing will become forced and chatting ceases to be an option. Pedalling feels like steady exertion.
Watch for a burning in your legs. If you are working hard enough to cause this feeling of lactic acid buildup, slow down immediately or you'll start sweating buckets. Switch into an easier gear, but don't stop -- you have to keep riding for the wind to keep you cool.
The down side of these approaches is that they're limiting. If you ride at an easy enough pace to stay bone dry, you won't be able to keep up with your riding buddies. If you want to get there faster, or get a better workout and still stay dry, you can do that too. The trick is to cool down before you get off the bike.
Your warm down should be a good fifteen to twenty minutes, or about five kilometers. If you cheat, and only warm down for the last couple kilometers, you will arrive all hot and sweaty, especially after a hot or hard ride. It's easy to forget, because the windchill is evaporating the sweat and keeping you cool, but when you stop you'll quickly begin to overheat. You'll get a feel for it based on climate and how sweaty you were during the ride.
The warm down should follow the guidelines in the previous section. If the ride was really sweaty, warm down such that the pedaling feels easy and you could chat without a problem. If it's dry or cool, you just have to cool down so that you don't start to sweat when the breeze from riding is removed, so adjust your warm down effort or time accordingly. Sit up so that your clothes can dry off.
If you want to stay dry, do not wear cotton. Cotton absorbs moisture and holds it to the skin, while advanced fabrics wick it away from the skin and evaporate it into the air. Many, many textiles are available which are softer, more durable, and faster-drying than cotton.
Clothes which hug the skin are best, but there are a surprising variety of ordinary clothes available which take advantage of advances in technology. Look for fabrics such as coolmax, polypropelene, goretex, lycra, and the like. These clothes will be perfectly comfortable both in the cold and in blistering heat.
Never wear more clothes than you need to when riding in the heat. A workout bra and cycling shorts are best, but don't wear long sleeves or pants, or anything over top of your clothes. Cycling shorts not only have a padded chamois in the crotch, but the rider doesn't have to wear panties, which makes sweating more comfortable. If you're not going for the spandex look, touring shorts work just the same but have pockets and a more everyday appearance, and any short made of advanced fibres will be a big improvement.
Buy some polypropelene socks, which are a big improvement over cotton. If temperatures are lower but the workout is hard, a soft wool offers some of the same features, wicking moisture away from the skin and remaining comfortable regardless over a wide range of temperatures.
Cycling is a lot more fun and comfortable when you dress wisely. Don't wear obsolete fabrics if you want to stay dry, and don't wear more than you have to. Store jackets and extra clothing in panniers or a handlebar bag, and toss the blouse on over the workout top when you arrive.
Do note that backpacks, camelbaks, and the like are notorious for leaving rider with sweaty backs. The better water packs store around the waist or use wicking fabrics to prevent this. With regards to cargo, from books to blouses, panniers offer a solution far superior to backpacks. The weight is stored close to the ground to improve handling and safety, and nothing is strapped to your back to leave that stripe of sweat that won't go away regardless of how much you warm down. There are tons of very practical designs for detachable pannier-backpacks and pannier-briefcases worth serious consideration.