Sorry to hear about the deaths.
As far as mirrors go, they are only as useful as you make them. Think about when you are driving a car. How often do you look in your rear view mirrors then? Does it differ based on time of day, traffic or road conditions? On a bicycle, it's the same issue - if you have a mirror and don't look into it, then it's just dead weight.
Other safety equipment you might consider includes: refelctive ankle strips, hi-visibility Sam Brown belt or reflective vest, safety triangle, and reflective tape strip on your bike frame/wheels/rims/spokes. Illuminite fabric clothing is excellent, in my opinion, but a bit expensive. Obviously, front and rear lighting is a must at night. While other riders love flashing lights on their bikes, I find them not only annoying but distracting - and in some jurisdictions they can be illegal depending on color and/or location on the bike.
Using only a single ear bud (or none at all) for music should help you hear approaching traffic.
Never riding two abreast on the road is something to consider.
Finally, always look for a "bail out" route out of traffic and/or off the roadway as soon as any vehicle approaches you from any direction. Even other bicycles can cause accidents.