How can drag induced by the landing gear be reduced?
The Pilot's Manual: Ground School has a nice diagram that explains it pretty well. It's all about streamlining. Airplane designers add landing gear fairings (I've heard them call "wheel pants") over top of the wheels and also on the landing gear strut. Beyond this, there isn't much else to do. The fairings could be made out of single pieces of material to limit the parasite drag introduced by rivets, etc.



There can be different variations on the fairing, but it pretty much comes down to fairings to reduce the drag:



NACA (precursor to NASA) details more in REPORT No. 518 THE DRAG OF AIRPLANE WHEELS, WHEEL FAIRINGS AND LANDING GEARS II-NONRETRACTABLE AND PARTLY RETRACTABLE LANDING GEARS

They also looked at wheel shape, but it primarily comes down to creating some sort of streamlined fairing to reduce the amount of eddying.