From the Genealogy Insider, 'Got the Picture? Using Your Digital Camera for Genealogy.'
I really liked this checklist of sorts from the Genealogy Insider on digital photographs for genealogy. The gravestone and record/documents lists are pretty close to what I do currently. For the gravestones, I also always take photos of landmarks (buildings, fences, walls, trees, notable stones) near the site/plot I'm visiting so that I'll have a visual reminder to help me find the spot again on a return visit. Adding a ruler to photos of heirlooms is a great idea, and I'm embarrassed that it hasn't occurred to me before this.