Why do I keep getting flats?
I own a bike shop and one of the reasons I noticed when a tube is punctured is that the rim tape/strip is not in place properly, like it has folded over itself.  If you have ridden your bike for a while, the rivets on the rim may have rusted and they create sharp edges that can puncture your tyre.  If this is the case, straighten the rim tape/strip to cover the rivets completely.  You will have to change the rim tape/strip if that is no longer feasible.  If you would like to save on paying for it, ask the bike shop to give you a punctured tube of size smaller than your wheel size.  You can cut a strip off the punctured tube and place it over the rim.  Make sure that you have cut a hole big enough to push through the tube valve.