You know how chilly the weather in San Francisco can be. This makes it great temperature for biking though. The air is cool enough I have minimal or no sweat to bike commute to my office.
A little preparation on clothing is useful. I usually wear just a T-shirt year round. This is a right balance between the cold air outdoor and the heat generated from the body exercising. It takes up to five to ten minutes to warm up to this balance though. So I usually start by wearing a jacket. At some major intersection I will hit a red light that stop for some 30 seconds. There I get off quickly, put down my backpack, take off my helmet, put off the jacket, stuff it into the backpack, and put the helmet back on. Usually the light will return to green at this point. So I right the bike, put my toe into the pedal, check that there is no traffic in my way and take off. If the timing is done right, this exercise will not cost me any time, definitely not a complete traffic light cycle.
Now that I think about my quick change drill, it is kind of like formula one pit stop! Not a second to be wasted.