The spring typically bends over or breaks off the attachment point on the mirror framework. It is easy to wrap some wire through the spring end and around the end of the mirror framework. By pulling with a pliers you can stretch the spring and twist the wire at the same time to get the tension back. You can see my DIY fix at
http://www.mbworld.org/forums/e-class-w210/335979-floppy-side-mirror-w210-fix.html I think you will find it to be an easy fix.