[RedDevil]

On the highway speed (or better, lack of it) is everything. On my 22 mile commute edging the posted speed limits (mostly 120 km/h, partly 100 km/h) would give me less than 18 km/l (42 mpg) while staying below 95 km/h all the way yields 22 km/l (over 50 mpg) both estimated under reasonable conditions.

Next big thing is tire size and pressure. Go for smaller rims/higher tirewalls, narrow thread and maximal pressure. The Prius thread shows that 17" low sidewall wheels can cause a 20% loss in MPG compared to 15" high sidewall tires.
Use only top quality tires, they maintain grip under high pressure much better than cheap tires.

Change the lights you use the most for HID's and LEDs if you plan to use the car for longer than 50K miles. Not a big difference in MPG, but it does help nonetheless and might improve your car at the same time; more light using less power.
Every kWh you save would save almost a litre of fuel. Using petrol and a car engine and alternator to produce electricity is very inefficient.

I went a bit further; I changed the 5W boot light for a 7W 24xpowerLED grid stuck on the boot light glass. It produces over 10 times the amount of light and it projects it straight into the boot, while most of the original boot light got lost in its housing (no reflector...!) so I have over 20 times more light in my boot than before.
Same with the 8W entry light. Great fun - when I unlock my car with the remote key the interior lights up so brightly that it lights the surrounding area too, enough to see any obstacles and such.