I have tried several things on my '99 5.4 and, while no exactly the saem, they are fairly similar.
The things taht do not work:
Cold air intakes - the Expy comes with a cold air intake and any after market unius do nothing except change the sound, and if you use the one with oiled filter, it will damage you MAF sensor.
Tuners - The will do nothing about fuel economy. If a tuner claims improvemsnts, ask him to give you a witten warranty...I bet not one will do this.
Premium gas - The car is optimized at the factory to run on regular gas and the computer will adjust accordingly. People that claim gains in MPG and power are just wishful thinking to justify the additional cost but, even if it would increase the efficienciy tiny bit, it would not offset the higher cost.
The things that work - keep your engine in good runnimg condition, change oil on schedule, keep your tires inflated to the recommended pressure (too high or too loww pressure will wear out your tires faster,) be sure your brakes operate freely and do not hang, and do not carry any more load thaan required (mine tend sto be a repository of tools).