I usually find the best way is to do 2 things
1) Break down most of the time like me, that saves quite a lot in fuel purchasing ways
2) put your foot down in 1st then go straight to 5th
A remap would certainly help the economical side of things. But then you can go down the route of tyres pressures etc. I think i read in a vw manual that if you can skip gears thats good too and making the gear change quicker is good.
I do what pete says. Best i had was 350 miles to a tank. I go through most places at 30mph in 5th.
Apparently a petrol that achieves 40mpg is considered good.