Exterior Rearview Mirror problems of the 1998 Toyota Corolla

Soon after purchasing my used 1998 Toyota Corolla in 2001 I had to replace the driver's side mirror. . . The mirror would not stay in position inside the "mirror-holder". I would position it, and a week later it would be at the wrong angle. This happened again and again. Now, three years later, the same thing is happening to the passenger's side mirror. I position it and a week later it is starting to tilt down. Unfortunately this time the mirror isn't covered under a warranty so I just keep adjusting it.