Void Undo

If you've used the <V> key to delete a section of the mesh and accidentally selected the wrong faces, type <V><U> right away; this will unvoid what was just voided. There is only one level of undo.

Fill Undo

If you've accidentally filled one or more holes that you didn't want to, use <V><G>. Holes are filled with green marked faces, so you can easily make them holes again by deleting marked faces.

Save Regularly

Use the Tmp 1 to 5 buttons to quickly save the current mesh into temporary files. If you make a mistake that cant be fixed in other ways, you can then recover an earlier version of the mesh from one of the temp save files. Regular saving is a good idea anyway in case of a software or system crash.

Ghost Mesh

This technique is better than a regular undo because it allows you to select which parts of an earlier save you want to restore back into the current mesh.

Lets say for example that you filled and sculpted an ear, touched up the nose, started work on the other ear, but then realized your work on the first ear wasn't any good. A regular undo, to get the first ear back to its original state, would also undo your work on the second ear and the nose. By restoring an earlier save file you would also lose your most recent work.