What kills O2 sensors? Running rich. What causes a rich condition? Lots of little things:

Short trips with a cold engine.
Air leaks into the intake manifold from vacuum lines that fall off or crack.
A bad MAF.
Engine coolant temp sensor reading too low.
Short trips with a cold engine.

You need to have your engine tested for vacuum leaks. And then have a monitor plugged into the diagnostic port, checking the O2 sensors, engine temps, etc.

If you only drive a few miles to/from work, do it in 2nd gear. That gets the engine warm, oil and coolant, and allows the valves to spin like the engineers intended.

Klaus
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Two great Volvos, a 1967 220 and a 1998 V70R