Well hell, since no one else is posting, I'll give it a shot.
Make sure you are using a fresh air filter. You may consider getting an oil-wetted filter from TRD or AMSOIL. They are very efficient, reusable after cleaning and re-oiling, and somewhat expensive. They are free-flowing compared with efficient paper filters and you can clean them as often as you like. I clean mine every 10k. The TRD filter is oil-wetted cotton and the AMSOIL filter is oil-wetted double layered plastic open cell foam. Don't put any more oil on the element than absolutely necessary.
Some people get additional mileage from opening up the air box (deckplate mod or removing the wheel well elbow from the airbox).
Tweak your AFM (on engines that have them instead of a MAF meter) as shown in:
http://www.automotiveforums.com/vbul...51&postcount=8
There is probably an optimum setting for gas mileage.
Your engine may get better mileage with higher octane gas, particularly if you do alot of highway driving. Use higher octane gas for a few tankfuls when you are on a trip, keeping track of the mileage and gas used, and see if that makes any difference. If not, don't waste your money. Say the price of 87 octane is 30 cents cheaper than 89 octane and you get 20% better mileage (from 15 mpg to 18 mpg) with the 89 octane. If the price of gas for the 87 octane is $3.00/gal, then you are paying 10% more per gallon, while saving 20% in gasoline usage. This is a good bargain. If you break even, I would still use the higher octane gas. You have to do the math and the experiment on your truck. At some price, it will be worth it for some percent increase in mileage. In all honesty, you probably won't see a difference in mileage between the octanes, meaning you should use 87 octane. However, it doesn't cost much to try and you should know about it if your truck derives a benefit from the higher octane gas.
Keep an eye on your ignition timing, idle speed, and wheel alignment. A high idle speed will cost you, as well as retarded ignition timing or bad alignment.
Buy road tires. Mud stompers will cost you. I believe that the more noise the tires make on the road, the poorer gas mileage they will give you. Fill the tires with 4-6 more psi than that recommended in your door label. Low tire pressure will cost you.
Don't fill the tires above the maximum psi labeled on the tire.
Lowering your truck a couple inches will give you better mileage on the highway.
Off-road driving will cost you. 4WD usage will also cost you, on- or off-road.
Installing manual hubs will make some difference. Each truck will be different in this regard. Manual hubs will keep you from having to rotate the half-shafts and a part of your front differential.
Keep your injectors clean with a fuel additive periodically.
Get any "Check Engine" light problem immediately. Many times you engine will default to a standard condition when a sensor goes bad. It gets you where you're going, but it is far from optimum as far as engine efficiency goes.
Exhaust headers and lower restriction cat converters and mufflers will help your mileage to varying degrees. Buy stuff that you find has helped other 4Runner owners.
I don't believe in cold air intakes. They are a waste of money IMO. Maybe some vehicles have poorly-designed intake systems and are helped by almost any mod. Yours is not one of them.
Fill your transfer case and differentials with 75W90 synthetic gear oil. Leave it in longer to partially offset the additional cost. The thicker the gear oil, the more drag on the truck. Lube your propellor shafts often for the same reason with Moly grease.
Use 5W30 oil in your engine - possibly 0W30 or 5W20. I have heard they work fine, but I have not tried them myself. Those lower viscosity oils will certainly make a difference in your gas mileage.
Maybe I'll make this a sticky - seeing how gas is getting precious. I'll also spread this around to the other truck forums once it gets more mature.