You've gotten good suggestions so far, and I can suggest a couple more things. First, get a journal style notebook and keep notes on your daily activities. This record can be invaluable as it usually contains more information than winds up in your reports. I have separate ones for each customer if I make regular visits to their shop. Next, get yourself a good quality camera with a large memory card, I prefer the ones that operate on AA batteries so I never have to wait for a re-charge on the job. I take way more photos than I need... just in case I need them later on. This will make sense the first time you (or your customer) needs a particular photo, and you have it in your archive.
(ProTip: Avoid taking a photo that might show something in the background that is none of your business, like a guy reading a newspaper at his workbench, or someone talking on a cell phone instead of working.) I have a padded pistol case that carries my camera, small tripod, extra batteries, paint markers, a small spiral pocket notebook, and some sharpies. Before each sequence of photos I use a page in the little notebook to record the item and date. This is really helpful when you are searching through the memory card. In addition get a good photo processing program, I use Photo Shop Elements but there are plenty of others. I mainly use the program to lighten or crop photos, add arrows, or add text.
An oddball tip is to buy a cheap 10' tape and cut it into 6" or 12" sections. These come in real handy for placing into photographs, measuring depths of excavations during a gouge out etc. You can even glue a magnet on the backside for taking overhead photos. You can trim the ends in a variety of shapes for different tasks for example a "V" shape to fit into an excavation, or nip the corners for safety.
My wife makes fun of me because I pack around a lot of stuff in the pockets of my work clothes.... and here is what I generally have: