Sometimes small pieces of glass,wire,etc can get lodged in the tire. Every time you get a flat you should run your fingers down the inside of your tire to check for these small pieces of glass, wire, or whatever else might cause a flat. Sometimes you cannot feel them so it's best to put some pressure against the inside of your tire and see if anything pokes you. Be careful you can cut yourself doing this. I've found a few pieces of wire and glass this way.