How do you protect outdoor sculptures from damage by grounds crews, landscapers, or mowers?

The best protection is preventative.  Make sure that artwork proposals take into account landscaping needs well before a commission is completed.  If a sculpture is surrounded by grass, then be sure that its base is protected from potential damage by weed whacking or mowing—this may mean placing it on a concrete pad, protecting it with rubber mulch (or a similar substance), or speaking to your maintenance professionals about proper precautions when working near the artwork.
If the artwork is already installed, then it may be necessary to add a “mow strip” or buffer that ensures space between the artwork and grass.