Smartphone apps like GasBuddy and some GPS devices can tell you the best prices at gas stations near you. You can also find the information online (Gasbuddy.com, for example).
When you get to the preferred station, pick the right fuel. Regular costs less than premium, and often won’t hurt your car even if the manufacturer calls for premium unleaded, according to TrueCar.com. If your engine starts to knock or ping, it doesn't like the lower-grade fuel.
When you're done pumping, be sure the cap is screwed on tight.