How can pilots minimize parasite drag?
Parasite drag is for the most part out of a pilot's hands, as it's a combination of:


Form drag: drag from an airplane's general shape
Interference drag: drag from airflow mixing around parts of the aircraft structure
Skin friction drag


You can reduce skin friction drag by keeping the airplane clean, dirt and smashed bugs cause friction, removing them reduces friction. You can also make sure all the little doors are flush. Other than that you can't do much without making changes to the airplane itself, for instance:


Flush fuel caps
Wheel spats: these aren't decorative, they reduce the wheels' drag by providing an aerodynamic cover
Seal kits: these reduce interference drag by smoothing airflow 
Cowling replacements
Fairing replacement


Whether these are worth the cost depends on the airplane, Cessna 152/172s are very draggy to begin with because of their rivets and wing struts, you could dump thousands into additions for very little benefit. Other airplanes get measurable improvements in speed from a few seals here and there.