Another test when you can't find the bugger is to 1) wear wraparound glasses or googles and 2) run an inflated balloon along the tire (and even along the inside of the tire if you can manage to flip it inside out). When the balloon explodes... well now you know where to look (and your eyes are protected).

Another thing I do if I have time on my hands (i.e. I'm not in the freezing rain) is get most of the tube out of the tire but leave the valve in, then inflate the tube just a bit. Then I look for the source of the little pssst! sound I hear, which gives me a hint of where to look for a shard or something in the tire.

Unless you've really beaten it, or it was really a bad quality tire, 1500 miles is not a lot for a tire. And even after there has been flats, it can be patched if there's a slash in it. (Depending on where the slash is and how it looks like, though, sometimes it can't be patched.) Don't overwear it, but 3000 miles would be a more decent range, and even more.

BONUS tip to avoid pinch flats: put some talc (baby) powder in your hands and run along the new tube before you insert it to make it slip more easily into place. And learn how to put your tire back on without using tire levers. There was a great thread on this board about how to best change tires (esp. without levers) and it has helped me tremendously, but I can't find it anymore. Anyone remember?

Grog, who just had four flats over the past month because the street shoulders are so full of grit and evil shards of everything