The two biggest factors for mpg are vehicle maintenance and driving style.
I'd start by running some SeaFoam through the tank and the brake booster vacuum hose to clean out any carbon buildup. Then I'd change the plugs. If there are any other obvious mechanical issues (bad sensors, noisy wheel bearings, clogged fuel filter, dragging brakes etc), get those fixed. And make sure all of your tires are properly inflated.
Driving style is actually the most critcal component for improving fuel economy. Rangers are extremely un-aerodynamic, so the faster you go, the worse your fuel economy will be. Watch your speeds, don't accelerate like every stop is a drag race, and don't be afraid to coast if you know you'll have to slow down or stop soon. Driving for fuel economy is all about conserving momentum. Don't go any faster than you have to, and don't race up to an intersection just to sit at a red light. Avoid using the brake pedal as much as you can and you'll be surprised with the fuel economy you can generate. (I get 31-32mpgs in my daily driver 2wd 4cyl truck).
Once the two previous things have been tackled you can address mechanical mods or aerodynamic changes if you want.
Mechanical mods can be things that reduce parasitic loss on the engine like the E-fan that RonD mentioned, or an underdrive pulley for the crankshaft if you can find them. An open intake air filter can increase fuel economy as well, but it's difficult to find stuff for an 06. A less restrictive exhaust may help as long as you're not tempted to push the pedal down more becasue of the increased noise. Or you could buy an aftermarket tuner from SCT and have a custom fuel economy tune written. Mechanical mods are great becasue they're always there, and many of them result in a bit more power in addition to the mpgs but they take a very long time to pay for themselves from a fuel economy point of view. For example, every 1 mpg I gain on my daily driver saves me about $70 a year, so if a new part costs $100 dollars, and gains 1 mpg for me, it's going to take over a year to pay for itself in savings. Your math will vary depending on yourfuel economy and how many miles you drive.
Aerodynamic changes like lowering the vehicle, adding an air dam for the front and/or side skirts, making a smooth underbody "belly pan" often have a larger effect on mpgs than mechanical mods, and they can be pretty cheap but they may not work for trucks that get used as trucks a lot, and some people consider them to look kind of goofy.