In _rough_ order from most important to least:
Proper maintenance : is everything working as designed ?
Cruising : stop and go will kill ya
Patience : keep it below 65mph
Correct shape : Golfs and Jettas are pretty aerodynamic. NBs are not. I'm guessing because they are tall and have flared fenders.
Lower rolling resistance : Inflate your tires within 5-7 psi of their max. Michelin Energy MXV4 Plus's use an extremely low rolling-resistance silica compound. Narrower and lighter wheels/tires are better, too.
High cetane fuel : better burning = more power
Full synthetic engine oil : less drivetrain loss
DRL disable : do you ALWAYS need your lights on ?
I'm sure there's more suggestions. These are just the ones I use, though I can't do much (now) about the shape of my NB