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2000 Corolla are very nice cars.
To get even better mileage - there is all the usual things to do.
Increase tire pressure, reduce unnecessary weight in your car, etc.
As per performance mods to increase mileage - an exhaust frees up the restriction so your engine does not have to work harder, instead of an air filter replacement you can get a SRI which allows the engine to breathe in air easier.
Keep in mind by making your engine more efficient, you will tempted to hammer down the accelerator thereby negating your mileage increase.