Well, today I did discover a puncture in tube, but no glass or anything within the tube. Then after spending all afternoon removing rear wheel and replacing tube, tire went flat again!
Someone looked at it and saw piece of sharp rock on outside of tire that did the damage. He commented tires looked worn (after about 1000 miles use). He put on a mountain bike tire and told me to keep it at 30 psi, but it felt wobbly to me like that. I think I prefer the hybrid type, about 50 psi feels right to me.
Any recommendations for good tires, superthick tubes, or whatever? Oh yeah, does anyone have suggestions for a good motorbike jack to lift the thing off the ground? Or a good stand for middle of bike that'll lift it at least an inch off the ground? That's why it took me four hours to change the tire.