My practices have enabled me to save time switching commuting modes from subway to bike and back:

Make a habit of emptying my bag every night. Lay out the things I need tomorrow whether I'm going to travel by subway or bike.

Wear clothes that lead to NOT needing a shower. The most important article for that is a wool t-shirt under my dress shirt. My workplace dress code requires me to have a collar on my shirt, so it's a given that I'll wear a woven, not-very-breathable dress shirt. I've found that a wool t-shirt under that wicks my sweat away without making my dress shirt wet. An added benefit of this is that the shirt doesn't need frequent washing. I typically wash it once a week, but I tested the limit, and it was OK after daily wearing for three weeks. The wool t-shirt is also appropriate for a wider range of temperatures than a cotton one is. It is not as hot as you might think in warm-ish weather, and it does keep me warm when the weather is cold, warm enough that I can often skip a sweater between my shirt and jacket. And it doesn't get soaked if I sweat heavily in the cold. A soaked t-shirt in the cold is a terrible thing.

If my face is sweaty upon arrival, I go to the men's room and wash my face and neck and maybe run water through my hair, too. Drying off makes me presentable, and the washing starts to cool my body down so I don't sweat from the rest of my body.

I wear wool socks year-round. Cotton socks can get wet, which promotes fungus on the feet, and when that gets bad, the fungus might stick with you for life. Never cotton socks.

Keep the bike ready to go, which might mean a tiny bit of maintenance every evening. Pump the tires if necessary, check the cleanliness and lubrication of the chain. Make sure the wheels are true and the brakes and gears are good. Brush off or wash off the worst dirt. I keep my commuter bike in my desk area of our apartment. I use spray-on furniture polish to wash my bike. The spillage on the floor comes right up with a paper towel or rag and doesn't leave a mess. Bike readiness includes having lights, a water bottle and a saddle-attached bag of pump, spare tube, patch kit, rag, tire levers, and maybe tools for minor repairs or adjustments. I might be more "prepared" than others, but I ride 14 miles each way and want to be self-supporting.

On the bike, I might carry shoes to change into because I usually use cleated cycling shoes. But if I don't want to do that, I also have a spare pair of work-appropriate shoes in the office.

I keep a spare set of clothes in the office in case I get soaked on the way in. I end up using this once or twice a year. It includes EVERYTHING, including t-shirt, dress shirt, pants, underwear, shoes and socks. Since I'm storing clothes, I might as well keep a jacket and tie for the very rare occasion that calls for them.

I rarely go shopping on the way to work or home from work, but to be ready for that, I keep a lock and home and a lock at work. I rarely use either lock, and I don't like carrying locks. If I unexpectedly want or need to go somewhere after work, I can grab the lock in my office.

A bit of thread drift: I also keep a spare helmet and bike at the office in case I arrived by subway and want to go shopping at lunch time or just take a quick spin apart from commuting.