my flats seem to come in pairs...I'll go a year or so with none, then in the span of a few weeks, will get a couple of punctures. riding right after rains seems to increase the rate too...washes crap closer to the curbs.

and when you're checking for things stuck in the tire casing, run your finger (or cotton ball or whatever) in BOTH directions before putting the tube in. I recently had a sliver of very fine copper wire embedded at a really flat angle that was only detectable in one direction...was completely unnoticeable in the other.