Lighten your right foot. Drive smoothly. Use cruise control when conditions permit on freeways (no major mountains in NJ so hills are not an issue).

I averaged 8.7l/100km (27 miles to the US gallon) with a 335i in mixed driving, including a friend who was determined to unleash 306bhp many times. I can get 21 miles to the gallon in mostly suburban driving in the X3.