What design techniques are there to reduce the drag of an aircraft and thus reduce the required cruise thrust.

Answer 1

Boundary layer air drag could be reduced by thousands of tiny little air bleed nozzles on the surface of the aircraft to reduce drag but is extremely expensive. Any design that reduces drag...be it mechanically or through some other means...is what's sought after. Wing root design is probably the most efficient method of reducing drag as well as elimination or modification of various structures on a plane and the hump-back design of geese has been tried on various aircraft to mimic nature's long distance fliers...in fact, there's a whole new science of studying structures and designs in nature to find very efficient designs.

Answer 2

The eddy current caused during drag are the largest enemy look here http://en.wikipedia.org/wiki/Lift-induced_drag#Reducing_Induced_drag Most designs incorporate a trailing member that can break up the acting vortex caused by the trailing edge of the wind, and mostly the tip.