Do a lot of reading. It's been said the biggest improver of fuel mileage is adjusting the nut behind the wheel.
Speed is the biggest killer of fuel economy, as well as quick acceleration. Have you put 50 psi in the tires yet? That's the easiest and most popular mod I think.
Check out the
4BT forum for giggles if you want. Lots of info there on putting in a 4BT.