Welcome to ecomodder.com .
A good start is to have a look at the hypermiling tips and start practicing as many of them as possible.
100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com
Look over the list every now and then, when you've already picked up some new habits, you can start trying out a few new tricks.
Depending on wether you want to mod your car, look over the modifications list.
Some of them, like adding a grille block and belly pan or removing spads can be done relatively stealthy if you don't want to ruin the look of the car.
65+ Vehicle modifications for better fuel economy - EcoModder.com
It's indeed not the most obvious kind of vehicle to start hypermiling with
