Drivers sometimes get a sort of tunnel-vision in which they're so busy looking for cars, they can overlook a cyclist in plain sight. As a cyclist, you can keep yourself safest by not relying on being just in plain sight, but by being in really conspicuous sight.
One big part of this is to act like the legitimate vehicular operator you are. Take the lane when you need it. Take the right of way when it's yours (while remaining appropiately wary of cars around you.) Yield the right of way when it's someone else's. Signal your turns. And don't duck in and out of sight in the empty spaces in parking lanes.
But your riding choices only go so far. Visibility is bad at night, at dusk or dawn, and in the rain. This is where lights come in.
In my municipality, it's illegal to bike at night without some sort of headlight (and I know people who have been cited for biking without it.) Here are some of the obvious issues in choosing one:
- its visibility (brightness and width of beam) to others
- how much it illuminates your path
- its battery life
And here are some more that are just as important, as they can make or break whether you actually use it:
- ease of keeping it powered
- ease of carrying it
- ease of putting it on and taking it off
The smallest of lights are little different from a small flashlight, and while they'll do something to improve your visibility, they won't do much to improve what you can see unless it's pitch dark. There are very bright halogen and LED lights that are very expensive, but make sense if you're going to be regularly riding at night. Halogen and incandescent lights burn through batteries very quickly -- you'll want to use rechargeable batteries (all the halogen models I know of come with a rechargeable battery; some incandescents don't.)
In general, if you ever park your bike in public, you don't want to leave any desirable accessories on your bike that can be easily removed, or they will be. So think about how you'll store it, and take it on and off (an easy thing for most models, which come with mounts the light can be slipped on or off.)
While some headlights are expensive, other things that can drastically improve your visibility are cheap. Reflective stickers can be put on your bike, helmet, bags, or anyplace else they'll reflect light from oncoming vehicles. LED blinkie lights are highly visible and last a long time on a single set of batteries (keep them in blink-mode -- they're even more visible and last longer.) You can get helmet-mounted blinkies, too.
Don't forget your side visibility -- you want lights (or at least reflectors) in all directions. (Again, my municipality has specific requirements about reflectors. Yours might, too.)
Add a reflective jacket or vest to all of the above, and you've done all you can do. You're now a really conspicuous sight, and that's a good thing.