Mashed potatoes seem simple, and that is exactly why they so often come out gluey or bland. In this video, chef Frank Proto demonstrates how to make mashed potatoes properly: choosing the right potato, starting them in cold water, and working the fat and dairy in gently so the texture stays light.
If your mash has ever turned to wallpaper paste, the culprit was almost certainly overworking the starch. Watch how little agitation he actually uses.