Product review (5 stars): use over air returns
These vent covers are meant to be used on the air returns (where air is sucked back to the central system), not the vents where the warm or cool air blows into the room.  In winter you can cover returns that are high on the wall, preventing already-warm air from being pulled back to the furnace.  In the summer, block off the lower returns to prevent cool air from being pulled back to the AC unit.  Since the air is being pulled in through the returns (as opposed to being forced out) these covers have no problem staying put.