If you have used Revit, you probably are well aware that it is a memory hog. I found that I can open a new project and zoom in and out and watch Revit grab more and more RAM. One method to get Revit to release that RAM is to simply minimize the Revit application, wait a second and then maximize it. You can verify this by opening 'Windows Task Manager' and monitor RAM resources used by Revit.
No one from Adesk has been able to explain this to me, including my new boss, Phil Read, but I thought I would pass it along. It has come in handy from time to time when Revit throws up the dialog indicating that RAM is low.