To clean and repair an AutoCAD drawing

http://knowledge.autodesk.com/support/autocad/troubleshooting/caas/sfdcarticles/sfdcarticles/How-to-clean-and-repair-AutoCAD-drawings.html

To clean and repair a drawing, follow these commands in AutoCAD: 1. Use RECOVER to open the drawing.
2. Unlock, thaw, and unhide all layers.
3. XREF and remove all attachments.
4. Select all and then OVERKILL.
5. DGNPURGE (not for AutoCAD LT):
DGN Hotfix for 2013/2014
DGN Hotfix for AutoCAD 2012
6. -PURGE All * N
7. -PURGE RegApps * N
8. -SCALELISTEDIT Reset Y E
9. Use FILTERS to delete all layer filters (must manually delete them in AutoCAD LT).
10. AUDIT Y
11. SAVEAS to save a new file or save over top of itself.

Additional ways to clean and repair a drawing are:
1. Use WBLOCK to save the entire drawing to a new file.
2. If 3D elements are present, use BREP to clear their object history.
3. Save the drawing to DXF format and then reopen and resave to DWG.
4. Use AECTOACAD to save a new copy of the drawing.
5. INSERT the drawing into a new file and then EXPLODE the block.
6. Use DGNEXPORT to export to DGN format and then DGNIMPORT to bring back to DWG.
7. Attach the file as an xref to a new drawing and then BIND the xref.
8. Save to an older DWG format, open in an older AutoCAD and then SAVEAS the drawing.