The best way to increase gas mileage is to adjust your driving habits straight up. The more MPG's the car gets stock, the bigger the difference will be from driving habits.
Pretend like your foot has an egg between the gas pedal and your foot, try not to break that egg. Remember that high RPM and load are what drives up fuel usage. If you just ease along allowing other traffic to pass you, you'll eventually get up to speed with them and spend much less doing so. Feel free to use higher gears, just don't give the engine too much work to do by giving a lot of gas while in those high gears. I routinely use 5th gear to keep going at 50mph, but if I need to accelerate I go back to 4th to get the job done.
Also think of braking as "negating the gas you invested to get going your speed". In other words, try to anticipate braking and do it as little as possible. When you get really advanced at this technique, you will learn to brake very early when there is stopped traffic ahead and go a certain speed that will be just slow enough to keep from catching up to the traffic and needing to all out stop, but just fast enough to not lose too much of your precious momentum.
Also something silly to do (don't really recommend this) is to take your windshield wipers off completely. They cause drag and could result in a very minor reduction in MPG. Not something I'd ever do, but if you're ever in a desperate situation like in your original post and need a no-cost solution, that's something you can try. BTW, always drive with the tailgate up if you have an open bed (no canopy etc). It's been proven having the tailgate down causes additional wind drag in the back, as strange as that may seem. Putting a tarp over the top of the bed supposedly may increase your MPG's but I have serious doubts on that one.