Not for everyone, but hard-soled bike-specific shoes, clipped into pedals, make riding so much more efficient and comfortable. They should be the second item of bike clothing to at least consider after a helmet.
Expect to be noticed
Whoever you are, remember that you naturally look great on a bike. And clothiers know this. Many (Rapha, Levis, etc.) are designing items specifically for commuters, such as pants and shorts with a higher rise in the back.
Forget the bell
Bells are highly useful, but riders who ping and then barge belligerently through are themselves a danger. Instead, be in control and prepared for any eventuality.
Metal tire irons
Regular cycling means the occasional flat, and metal tire levers for removing the tire from the wheel rim are far superior to plastic one. And yes, learn how to fix a flat.
Share the joy
Cycling is a bonding experience. It's that rare endeavour in which acknowledging others of like mind, however subtly, is always apt.