A Geo Storm should be capable of excellent mileage. The first step is to examine the driver and the driving.
Does the driver accelerate at full throttle and rush up to red lights to slam on the brakes? Are they a left foot braker who rides with their foot lightly on the brake pedal?
What kind of driving is being done, is it short trip city driving and do they idle their car a lot or maybe let it warm up for 20 minutes while they drink another cup of coffee?
Then the car, are the tires properly inflated, the brakes and the parking brake checked for dragging? Is the wheel alignment way out of spec?
Do a compression check on the engine, if the compression is bad, no amount of tuning will help.
Is the transmission or clutch slipping?
Are the fuel injectors dirty, it the engine operating at the right temp and is the check engine light on?
A dirty air filter will hurt your power, not necessarily your mileage.
A properly designer cold air intake can give you a little more power, but if you use it it would actually drop your mileage.
New plugs might help, but only if they are needed.
In short there is no secret magic answer.