Keep track of your mileage – write down the mileage for each tankful (using the trip odometer) and the amount of fuel you pump each time.
My Scion xA is rated 30mpg in that Combined (2007) EPA rating, so I was over 50% above this last summer, and during the past 90 (winter) days I have been 43.3% above.
The front grill blocks and fog light covers added ~10-12%, and the smooth wheel covers added ~6%, and yes the rear wheel skirts are ~2 to maybe 3%. The rest is gained from ecodriving technique. I use a ScanGauge II to help me see how I am driving, and to help me practice and improve my ecodriving.