It may sound like common sense but you'll want to consult the weather forecast before you head out. If you forget to wear a jacket when traveling by car, you may hardly notice the unpleasantness during stops to look around. But when you're exposed to the elements for hours at a time, you'll wish you'd been more observant of Mother Nature's plans.
Once you know what's in store, you can pack your clothes accordingly. If you're looking at mostly warm and dry temperatures, then you're safe with a biking jersey or a similar lightweight top. But if your ride will last into the evening or night, pack a lightweight jacket you can easily slip on if needed. Keep in mind, however, that your body will heat up as you exert yourself on the bike and clothing that you don't use will have to be carried [sources: Independent Traveler, GORP].