How I get better Mileage

I have a 1999 Mercury Villager minivan. It was getting about 17 to 19 MPG depending on how much highway or City driving I do. 1 week before I went to my Sister's house in NC, I got a total transmission fluid flush( Cost $99.00), I also put in Amsoil 5w-30 oil with the Amsoil oil filter. I waxed the the Minivan, put RAin X on the windshield, installed a new Fram Air filter the day before I left. I got 28 mpg on the first tank, 27 mpg onn the second tank. third was 27 mpg Forth was 26 mpg. Avg highway speed was 75mph. Braking was light, But heavy in Baltimore.
This test was done 2 years ago. I am getting 21 to 23 MPG mixed mileage now. I just had my transission fluid flushed 2 monthes ago. before the flush, I was getting 19 to 21 MPG. I running mostly Mobile 1 once in a while I will buy Amsoil through the internet.
Now, I do think the Amsoil has had some effect on the mileage but, Changing the transmission fluid is were I picked up the better mileage. The Villager has about 164,000 miles on her. Tire are important ( The right air Pressure), Finding the right gas grade( For me Reg gas) using the right Oil. But everyone forets the transmission. I have used Amsoil or mobile 1 for a while now. Both are good. Mobile 1 will go on sale were as Amsoil doesn't. And Mobile 1 is easier to find.
Now, can a Mechanic make sence of why a transmission flush will increase mileage?:banghead:

ITS ALL IN YOUR HEAD... j/k

maybe the flush made it easier and smoother to change through gears causing less work for your tranny/engine.
i think that would increase mileage....

but its a shot in the dark.

Oils only have a very limited effect on fuel consumption, for example energy conserving low viscosity engine oils show improvements in the order of 1% compared to a reference oil. Mobil 1 low vicosity oils are generally classed as energy conserving.

The biggest effect on fuel consumption is your driving habits which can effect the consumption as much as 30%.

The flush won't have any significant effect.

I can see a good filter and clean passages creating better overall flow through the transmission which translates into more response from the torque converter, but I do agree with SaabJohan about a marginal increase in milage.
If you want to up it more, change your oxygen sensor at 20,000 mile intervals for peak response, and keep your spark plugs in peak condition as well. You could even index your sparkplugs to create the best possible flame front within the combustion chamber. It sounds like you have a good handle on it though.