1) Look where you're going, and be aware of hazards all around you.
2) Ride defensively. This means claiming some road space rather than squeezing yourself against the edge of the road. Other traffic can wait if necessary.
3) Keep your bike maintained properly. You'll have a less stressful and safer ride if everything's working properly.
4) Master the use of the front brake. It's the most important and can give the shortest possible stopping distance by itself if used properly.
5) Use a good set of lights at night. Reflectors aren't enough.
6) Wear protective equipment if you feel it appropriate. Gloves will give some hand protection if you come off, when you'll instinctively put your hand out to break your fall. A helmet will not prevent concussions, nor will it help if you get flattened by a bus, but it can reduce or prevent minor injuries.
In my opinion, protective equipment is the lowest importance of these factors: far better to prevent the accident by good riding technique than to have to rely on safety gear. There seems to be a common assumption that wearing a helmet is the be all and end all of safe riding. It isn't.