- notice system starts swapping (if you do not monitor that, to me it sounds as careless as driving on the highway on 2nd gear and ignoring engine noise -- ideally the OS could proactively help here but unfortunately I don't know a good "automated" tool)
- find out which process/app uses the most memory (Linux can even tell you which ones use the most swap space [1])
- decide which one you want to (gently|forcefully)-(quit|restart|whatever). Exercise judgment.
[1] http://stackoverflow.com/questions/479953/how-to-find-out-wh...