What would you do to make a plane environmentally friendly?

Winglets. These are extensions to wing tips that curve up into the air about 1 foot, and were developed by NASA. They enable an aeroplane to achieve cruising altitude faster, thus reducing fuel burn. Also, with jet fuel prices soaring last year, many airlines have already implemented their own fuel-saving programs, such as taxiing on one engine only, and letting gates do push-backs instead of using engine power for that. Also, filling more seats on flights, and reducing baggage allowances from 70lbs to 50lbs. Flying point-to-point instead of via hub-and-spoke, and choosing 2-engine jets, such as 777 or 330, instead of 3 or 4 engine-jets on long-haul routes.