Lots of people are of the opinion that AutoCAD 2009 is about as memory-watertight as a rusty sieve with a hole punched in it, and it’s certainly a major RAM hog, so it was interesting to read this:

First open your Task Manager… switch to Processes and sort by Mem Usage.Then open a large drawing in AutoCAD 2009 (any flavor).. choosing one with several viewports make it even more interesting.

Once opened.. Minimize the complete AutoCAD session window (not just the drawing).
ACAD Memory Usage drops almost 90% once the window is minimized.
Now Reopen the minimized AutoCAD window and the memory returns to a workable level.
The ACAD memory still does a serious memory ramp up during any saving function which is still a problem for us.

This technique seems useful in reclaiming or resetting AutoCAD’s memory usage without restarting.

From here: http://discussion.autodesk.com/forums/thread.jspa?threadID=702176&tstart=0