Drive like a Grandmother? LOL, the plugs/wires could help more then you would think. Elimenating as much drag (friction) as possiable is also a good step, change ALL your fluids(tranny, oil, transfer case, both diffs), use the best fliuds you can afford. Anything that reduces friction it going to help your MPG, and if some of your stuff is still stock fluid it could be gumming you up quite a bit. Grease everything that takes grease, check for drag in your wheel bearings, same as with your A/C compressor, make sure they move freely.
Not a whole lot you can do with your motor, not like you could on old cars. Maybe check to make sure all your sensors are working correctly. Clean everything you feel comfortable cleaning. Make sure all vacuum lines are in good shape. I'm sure there are many other things I don't know about, but these are the general things I've trouble shot in the past on my many, many cars. Maybe you'll get some other idea's from people more experenced with the explorer then I am, I'm still only on my 2nd month of ownership, even though I've done enough work on it already to account for 10 years of ownership! LOL