Sun visors do more than block the sun these days, with the added weight of GPS units and CD cases weighing them down and eventually causing them to droop. This looseness is a fairly simple issue and you can usually fix it in a matter of minutes with basic hand tools.
Sun visors are generally flat pieces of fiberboard wrapped in vinyl, plastic, or other material. Visors are attached to the interior roof of your vehicle at two mounting points. One is a fixed mount where the visor snaps in and out of place and the other is a swivel mount that allows the movement when you raise or lower the visor or move it to shade the side window. The mounts are nearly always the culprits of a droopy sun visor.