I'm moving this to Bicycle Mechanics (from General Cyclin discussion).

Have you checked you have enough pressure in the tubes? When you hit a bump, underinflated tyre compresses too much, and the tube gets pinched against the rim. These are called pinch flats or snakebites, and are fairly easy to spot because they usually result in two small holes close to each other. I would imagine your dealer would have noticed by now, if this was the problem. Nevertheless, make sure you inflate the tubes up to the suggested pressure (it's printed along the outer tyre wall).

Pinch flats aside, all repeated flats that I've ever had have resulted from something sharp (a small piece of glass or sand) lodged in the outer tyre and then slowly working its way through the tyre towards the tube. Next time you have a flat, make sure you (or your bike shop, whichever fixes the flat) know where exactly the puncture is in relation to the rim/tyre. That will tell you whether to look at the rim or rim tape, or the inner surface of your tyre. It will also tell you where exactly in the rim or tyre you should look at.

Finding the culprit in the outer tyre is difficult unless you know the exact spot. Even then you may not find anything wrong on the inside of the tyre, but looking at the outer surface you'll see a small cut where the sharp piece entered. If there's something wrong with the rim or rim tape, that's usually easier to spot. Still, it helps to know where to look.

--J