On my Tempe/Chandler commute with the old commuter on skinny tires (700x28) I would average maybe 2 flats a year (2000 commute miles). I was careful of where I rode and kept an eye out for the goathead thorn weeds growing in the seam between gutter and road to try and avoid those areas.
Now I have a fatter tire commuter originally with 700x45 Specialized Borough and now Speciailzied 700x42 Sawtooth tire, haven't had a flat in 2000. I am much more careless about where I ride and even hit some trails on South Mountain occasionally as part of my commute. Heavy duty flat resistant tires really make a big difference if you don't mind the weight and rolling resistance