Tombstone rubbing and chalking are bad for the stones. However, an acceptable method is tinfoil "rubbing." This is where you place a piece of foil over the headstone and use a soft brush to create an impression. Overall, this technique did work, but it was somewhat frustrating and not worth the effort. It was difficult to get a photo of the shiny foil and shiny side up seemed to create the best results. Here's a photo of John P Quattlebaum's headstone, which isn't impossible to read, but not much easier either:
Another suggestion was to wet the stone, which would give it a more even tone. This is the method that I found to work best. We took a spray bottle, as well as a gallon jug of water. Take this stone for example, which is pretty much impossible to read:
Now see it what it looks like when wet and from an angle:
What a difference! Now you can see that this is a stone for Ben and Permelia Ouzts: Together They Dwell In Our Father's House.
Overall, I'd say that wetting the stone worked better. I'll be taking water with my whenever I go out to older cemeteries from now on. I'll probably leave the tinfoil at home.