Fuel economy gains come from unglamorous basics far more reliably than from any gadget or additive marketed to "boost mileage."
Under-inflated tyres are one of the biggest silent fuel-economy killers — check pressure monthly against the sticker inside your door jamb, not the number on the tyre sidewall (that's the maximum, not the recommended figure).
A clogged air filter forces the engine to work harder to draw in air, richening the fuel mixture unnecessarily — a fresh filter is one of the cheapest fuel-economy upgrades available.
Aggressive acceleration and hard braking waste far more fuel than driving smoothly at a steady pace — anticipating traffic ahead and easing off early instead of braking hard is a free habit change with a real impact.
Remove unnecessary weight from the boot, and close windows at highway speed (aerodynamic drag from open windows costs more fuel than air conditioning above roughly 80 km/h). Finally, stick to your service schedule — a poorly tuned engine, worn spark plugs, or low fluid levels all quietly drag mileage down over time.