There are a couple of things I find invaluable to my work and I keep with me everyday.
The first thing is I carry a pen, usually two. I prefer Pilot G2 pens. Everyday, I have a black one with a .38″ tip and a red one with a .07″ tip in my pocket. If you’re less picky, just keeping one with you will prevent the always having to ask to borrow one or to scramble to find one when needing to make a note for yourself or leaving a note for your customer.
Second is something to write on. I like to keep a small notebook in my pocket. I prefer the Field Notes books because they hold up well to the daily use and they’re small enough to be weightless and they can get bent up and have pages ripped out without losing their binding.
I also rely on Post-It notes. I keep a few in the inside of my notebook but I’ve known other techs to carry a couple in their pockets. They are perfect to making or leaving notes. They’re easy to replace and I keeping them inside a notebook keeps them in good shape.
I never leave my desk without pen and paper because of the number of people I see everyday combined with my terrible memory, I would never remember to do half the things I agreed to do or remember what I did to fix an issue by the time I got back to my desk to document it and make notes in the ticket.
I carry a couple of USB keys with me as well. I have a larger capacity drive for utilities and software to install. In addition to documentation and space for backing up files, saving screenshots and other storage needs.
I have a second smaller capacity USB key I keep with bootable tools on it. There is a wide choice of portable applications that are perfect to keep with you and can be used to boot into or run from the drive.
Working in customer service has reinforced the only certainty is uncertainty. I never know what I will see from day-to-day. I never know what challenges will be thrown at me and what tools I will need to do my job, so I try to keep a variety of tools at the ready.