When I visit a cemetery to photograph my ancestors gravestones I try to do it in the late morning or early afternoon. The sun is usually at a better angle at this time of day for photographing the stones. Just be sure not to throw your shadow onto a stone with a glossy surface or you will see yourself in the photo. (Yeah, I did this and, unfortunately, did not discover it until after I got home.)
I prefer to use a digital camera with a zoom lense and an LCD monitor. The monitor lets you know immediately whether or not you will be able to read the inscription. If it is an older, hard to read stone be sure to write the information down in addition to taking the photograph. In fact, I do this even when I can read the inscription in the monitor just to be on the safe side. After all, if I have traveled a long distance to get the photos I probably will not be going back anytime soon, if at all. Better to be safe than sorry.
With a large gravestone you may not be able to read the inscription clearly in the monitor, or on the photograph. This is where a zoom lense comes in handy; you can zoom in and photograph the inscription, in sections if necessary. Be sure to also get the full view of the stone.
Sometimes it is helpful to take a photograph at an angle in order to have some shadow in the inscription, which makes it easier to read. It depends on how the sun is hitting the stone. Be careful also with using a flash. A lot of the time it causes too much light and fades out the inscription making it unreadable.
Those bronze plaques that lay flat in the ground are harder to photograph well. It is difficult to read the dark lettering against the dark background. I have found, though, that you can deal with this in whatever photo program you have in your computer. I used Paint Shop Pro to lighten a plaque photograph so it is easier to read.
Another thing I like to do is make a sketch of the layout of the gravestones, especially when a family is grouped together. In the sketch I draw the shape of the stone and put a number on it; then further down the page, or on another sheet of paper, I write that number along with the inscription and a brief description of the stone. I also take an overview photograph showing this same layout. This can be very helpful if you ever need to go back to the site or another member of the family wants to visit the site.
Don't neglect the stones in the plots next to or near your ancestors. Take photographs of these stones and closeups of the inscriptions even if you don't recognize the names. You may find out later that they are extended family.
Take the time to stop in the cemetery office and look at the records. Sometimes there is more information than what is on the stone - such as a baby buried in a plot with another child or adult, or the name of a relative who is/was the owner of the plot. Also, if you can't find a stone for an ancestor but you know they were buried at that particular cemetery, the office can tell you if there was no stone at all.