I've heard and read of all kinds of gimmicks but I know of three things that absolutely will cut your mpg.
1. Feather it. It's the most difficult because it's behavioral but it Costs $0.00 and will cut your mileage down a few mpg. Some say more, some say less. But it will cut your mpg.
2. Weight. Shed lbs. and you'll improve mpg. Mathematically, it's the best way to increase your mpg. 3rd row seat. Junk in the trunk. Spare tire. If getting mpg is important to you, drop weight any and every way you can.
3. Drafting. If you're lucky enough to find a box truck or tractor trailer get behind him. You don't have to ride his a$$ but anywhere from 12 to 18 feet will improve your mpg. Did this as often as I could with my wife's Honda (yea, don't ask) and got her MPG up from 34 to 38 mpg - that's a full-sized Accord.