The best weight loss trick is to simply drive around with no more than half a tank of gas. Its the cheapest and easiest modification you can do.
I don't bother with weight reduction though. My car routinely reports between 40 and 46 mpg with zero modifications and a full tank of gas. Take into consideration that I am also running a different size tire than OEM. In all honesty, my mpg is slightly better than what the computer reports but I am too lazy to do the math.
Shoot, I've even seen the computer report 55 mpg on days where my speed stayed in the 50 mph range. I figure, with gas mileage like that, whats the point in hyper-mileing or doing any modifications?
I simply fill the tank with gas every morning and do my 200 mile drive every day.