Driving habits are free, so they automatically return the biggest bang for your buck
They're usually where the largest initial gains come for people new to the efficiency game.
A few years back, I tried to quantify the gains from driving technique vs. mods. At the time I came up with 38% from driving, 28% from mods (the ones I had tried up to that point, anyway).
2006 retrospective (there's more where that came from) - MetroMPG.com