Tune up your bike and always wear a helmet. Use a mirror when riding. Shift for hills.
Most people forget to change gears when climbing or descending small hills.
Trying to ride in too high of a gear is the fastest way to ruin your knees and the week. Pedal down hills. Your bike will be stable if you are pedaling, not just coasting, down a hill. Always descend in a high gear to retain the ability to accelerate if the situation calls for it. Use both brakes.