The best and least expensive things to do are; Regular tune-ups, use quality high-flow air filters, correct spark plug gapping, check your belts for correct tension and wear, Check and replace worn belt tensioner and idler pulleys, regular oil changes with a high-grade synthetic oil and a premium oil filter, front end alignment with properly inflated tires and check your brakes to make sure they are not dragging. Remove any un-necessary tool-boxes and cargo items that will add extra weight. Good driving habits will also go a long way in keeping your mpg's up.
