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Start with instrumentation. If your car doesn't already have a fuel economy readout, you can make one (MPGuino) or buy one (ScanGauge or equivalent).
Then start reading through the list of driving techniques and working on that. It's very rare that someone can't improve their economy quite a bit by modifying their driving style.
100+ Hypermiling Tips
After that, you could start looking for ways to modify the car itself.
65+ Efficiency Mods