Maybe one flat per 2000 mi ridden? Estimate 2/3 due to wire (from steel belted car/truck tires?), 1/3 due to glass. Glass is usually easier to see, those wires are invisible.
There are a few things you can do to somewhat reduce the risk of a flat: keep an eye out for glass (looking down the road far enough so you can plan evasive maneuvers), avoid riding through the "debris triangles" in intersections; avoid riding through piled up sand/dirt (glass and those little pieces of wire get washed off the road and trapped there, waiting to pounce), absorb bumps with your legs/hands (though I've only had one pinch flat), once you've located the hole in the tube, look carefully for the cause in the tire and don't believe that it fell out - keep looking, be gentle with your tube valves, etc.
I get somewhat fewer flats now than I did as a newbie, but they still happen.
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