Protect your head. Studies show nearly all bicycle-related deaths involve head trauma. Wearing bicycle helmets can reduce the incidence of head trauma. Wear clothing that makes you more visible. Clothing should be bright colored, and tight fitting to avoid being caught in the bike's moving parts.
Reflective tape can also help.
Stay out of drivers' blind spots. Carry books, lunch boxes and other items in a bicycle carrier or backpack - not in your hands. Go with the flow of traffic - not against it. Riding against the traffic puts you where motorists don't expect you. Ride far enough away from the curb to avoid the unexpected from parked cars, such as doors opening.