Incidentally what to modify on your car comes down to three questions:
On a scale of traffic jam to highway, where do you drive?
Traffic jam: Mods are geared to weight reduction and engine being off whenever not required. Extend your coasting time as much as possible. Kill switches, lightweight flywheel, wheels, empty the trunk, underdrive pulleys (always a good idea for efficiency, only real concern is alternator not producing enough power at idle. But a real hypermiler never idles anyway), carbon fibre everything etc.
Highway: Aero, gearing, and lean burn. Undertray, lose the factory rear spoiler, ditch a wing mirror, wheel covers/skirts (the general shape of a prelude is pretty good actually, it's the details that kill cars of the 90's), get your computer tuned to make lean burn a reality. Tallest gearing you can get.
How much money have you got?
Ever seen a coroplast undertray that can take a scrape? Nope, but ABS plastic can. Genuine moon hubcaps v pizza pans. Get an aftermarket computer (or re-tune) to take care of injector cut-off and trans control for you while us plebs deal with toggle switches and buttons on our gearsticks. Carbon fibre doors.
How crazy are you about fuel consumption?
Basjoos