Some tricks that I have found to save fuel:

• Instead of using air conditioning, use cold defrost which gives you air conditioning but doesn't work on the same duty cycle.  It can take a while to cool down the cabin in stop and go in the summer but on the highway this really saves
• Ridge ride on the highway to run on the high spots of the lane on a worn in highway road
• If you drive divided highway, try to side draft if possible without freaking out others
• Don't run ethanol blended fuels since it drops fuel efficiency and raises ground level ozone levels and it's corrosive as per Jay Leno
• Watch spark plug gaps and index them so they're in the same orientation in each cylinder
• Keep the mass air flow sensor clean (isopropyl or electronic contact cleaner will work too)
• Replace your upstream oxygen sensor(s) if they have over 80K on them
• Keep your air filter clean
• Keep the throttle plate in the throttle body clean - use seafoam or throttle body cleaner with an old toothbrush
• Seafoam the intake every oil change and use it in the engine as a flush BEFORE dropping the old oil and the rest in the gas tank (1/3 in each) - DO NOT RUN SEAFOAM THROUGH THE INTAKE ON TURBO'D APPLICATIONS!!!
• Use the lowest viscosity fluids that meet your required fluid specifications - Amsoil Low Viscosity ATF is great for automatics that can use it and for vehicles off warranty try Royal Purple engine oil with Synerlec
• Flush your hydraulic power steering system and use synthetics if available and run the return line to a catch pan/bucket.  I would also add the guy behind the wheel on the key should work the steering lock to lock during the flush while the engine is running
• Replace differential oil with synthetic low friction gear oil like Royal Purple and use the proper grade
• Don't fill the engine oil completely full to control windage - aim for the middle between add and full but beware some dipsticks (Toyotas) have the wrong markings on their dipsticks - check first online
• Watch tire pressures
• Invest in an OBD2 bluetooth adapter that works with Torque Lite on Android devices to watch your oxygen sensors and other real time data like fuel trim, % throttle, and mass air flow
• Watch the weather and try to drive with a tail wind or at mid day when the air is the least dense at its warmest
• Check brakes for dragging and inspect wheel bearings
• Have alignment done as close to zero toe in as possible without hitting zero - electrics do this and some hybrids
• Choose the lightest tires with low rolling resistance and wheel combination within your price range (tirerack.com can be a good source) because reducing 1 pound of rotational mass is the same as removing 5-10 pounds of static weight
• Check the PCV regularly and use synthetic engine oil so it doesn't gum up or get clogged - certain engines will cook conventional oil until it turns into sludge (Here's a list)
• Put 1/2 tsp of ws2 in the engine oil, 1/4 tsp of ws2 in the coolant to lubricate the water pump, and 1/4 tsp of ws2 with fuel stabilizer in the gas tank to lubricate the fuel pump and injectors.  You can also find branded additives from Archoil with their AR9200
• Run rare earth magnets on the oil filter to help trap metallics in the filter so they don't circulate in the system and use a good quality oil filter
• For vehicles tuned for 91 octane, it may help to use a mild octane booster. I have had great results with Millers Petrol Power Ecomax using 1 mL/L and STP Octane Booster using 2 mL/L. I am also having success using 1 tsp of raw tungsten disulfide (ws2) mixed with approximately 150 mL of Lucas Fuel Treatment with Upper Cylinder Lubricant per 60L of fuel.
• Consider installing LED based headlights like the Cree-based 3000 lm LED bulbs since they save fuel proven by Audi and the EU