It depends a lot on why the ceiling is sagging to what methods are best used to either prevent further sagging or repairing.
It sounds like you have a fibrous plaster ceiling.
There are a number of threads on this board that have many suggestions for fixing and repairing sagging ceilings. Try this one
http://www.woodworkforums.ubeaut.com...ad.php?t=49940
Cheers Rod