Driving style will do it, for sure. My best in-town drive is 77mpg (scangauge), and my best highway is 75.2 over 150 miles. This is using all the tricks, including Pulse & Glide. Read the articles at CleanMPG for details. I'm not sure you'd want to go that all-out across the country, but at least keep your speed down.
Lower grill block is a good suggestion, to a point. On our cars, only the right-hand (passenger) 1/2 of the grill actually has engine radiator. You can block off the other side if you don't need the AC. With temperature monitoring (scangauge), you can block some of the right side. It'll help with aerodynamics. For a temporary block, you can use duct tape or clear packing tape. Also look at covering the upper grill. Not much airflow through it, but an aero disturbance still.
did the 1600 plus miles averaged 40-42 doing 75-80, I hated being in a hurry, but hurt my back and every extra minute was hell. Dropped to 65 and got 43+ but was hitting big time wind. I think if I did 60 I could have gotten 45mpg, tires at 35lbs, no wind, no a/c, nice trip, and sure beats filling up my Avalanche at 90 bucks a fill, it was nice to do 300 miles at 7 gallons of gas!!! Frank