The art of getting as many runs on virgin snow as possible.
Powder preservation is most often practised by skiiers in the backcountry where each trip down the hill is earned by a long hike up. Getting the most bang for your buck is vital in these situations.
Skiiers who do not follow the basic rules of powder preservation in the backcountry have been known to have strange accidents befall them whilst skiing with more well behaved skiiers.