I agree with SkyPilot. Start off with those tune ups as they are easy and quite cheap to do with the exception of probably the wires and o2 sensors if they need to be replaced and installed. Furthermore, if you're not already doing so, use synthetic oil. This helped me gain 1 - 2 MPG extra. Don't forget to make sure that you have enough coolant in the reservoir too.
Another extra thing you can add to that list is to rotate your tires if you haven't done so in a long time and make sure that the tires are inflated up to the recommended pressure (check the door panel for the specs).
15MPG is really low for a focus. Good luck!