I like the "Don't go over 2000rpm" suggestion.
Of all the things I have tried in my truck, that is the only one that works.
I think the fact of the matter is, DC engineers have designed these trucks to be optimal performers. Changing your intake or changing your muffler or any other physical modification you do, is probably moreso for sound than MPG.
It doesn't matter how much money you throw into your vehicle, if you drive around all day with the pedal to the floor... You're going to get about 11MPG. No way around it.
I found in my own personal tests, that city driving in 3rd gear at 1500rpm, I got about 15.5MPG. That includes some stop and go traffic, but always keeping my acceleration under 2000rpm and my steady driving speed at 1500rpm.
As already stated, for highway driving... If you're in no big rush, do the speed limit. I found that over approx. 750km (466miles) on a 26 gallon gas tank... The overhead computer reported 20MPG. If you do the calculation by hand, it works out closer to 18MPG... Regardless, 18MPG is more than acceptable for a quad cab 4x4 with 500lbs of passengers.