If your looking for an immeadiate and low cost fix there really isn't anything other than slightly over inflating the tires and driving style. The more you push that brake pedal.. the more gas you've wasted.
Now if your in this truck for the long haul and want to save money **in the long run**.
Crank underdrive pulley, Tune, rear gears, & Reducing weight are the top four that will pay off in time. But your looking at 75k+ miles for them to pay you back in savings.
From a milage only standpoint.. e-fan is not worth it, Exhaust is not worth it, CAI is not worth it
For what it's worth.. On my 06 FX4 I've done, K&N CAI, aftercat magnaflow, Bama X-cal, crank underdrive, e-fan, & run my stock tires 3-5psi over inflated. I drive easy most of the time in 50/50 city/hwy traffic. Given all this I average in the 21-22 area. It'll get up around 23-24 on freeway trips too.
But I've also spent a lot of money getting here. And since I plan on running this truck to 150k+ miles it will "pay off". Honestly though, milage improvements are only 1/2 the reason I did all that.