I keep a customer list (names, numbers, addresses) in my phone that's updated with new accounts once a month. It gets updated on the computer at the office, and then emails it as a PDF to all technicians/crew members. As for stuff on paper, I have a list printed off the computer once a month that shows the balances for all lawn accounts.
If I need to write notes and customer info down while I'm in the truck, I have a calender book for that. It's one of the ones that shows time slots (per hour) for each day. If I don't use that, I just use the memo pad app on my phone.