It's a beautiful Saturday morning and you're planning on cleaning some ancestors' gravestones in a little rural cemetery two hours away.  You run down your list of needed items:  Scrub brush?  Check.  Plastic scraper?  Check.  Denture brush?  Check.  Wood inscription stick?  Check.  D/2 Biological Solution?  Check?  Sunblock?  Check.  Bug spray?  Check.   Seems like you've packed everything you'll need to safely and effectively clean some gravestones, right?   Just don't forget the WATER. 

Over the years I've heard from several folks that they had neglected to find out ahead of time if the cemetery they planned on working in had clean water available on site.   Water is the most important item needed for cleaning a gravestone, and you need a lot of it.  How much?  I would suggest a minimum of one gallon per cubic foot of gravestone.  So, if you plan on cleaning a 3' wide x 3' high x 1' deep grave marker, I would have  nine gallons of water.  Again, that's just the bare minimum.  More water is better.  Keep the stone wet while you clean.

2)  What if the cemetery in question appears to be abandoned or neglected?  Must I still receive permission to work in the cemetery?
Answer:  Yes.  If you suspect that a cemetery is abandoned or neglected, contact the Office of State Archaeologist.  If the cemetery in question is in fact unrecorded, then one of these state agencies typically is the authority on granting permission to work within the cemetery.  Regardless, you still will have to request permission from the land owner to access the cemetery.

[note: The above answer comes directly from the Wisconsin Historical Society - Historic Preservation Division.  If you're not cleaning a gravestone in WI, check with the state historic preservation office in your state because similar rules will apply]