Ooops: one notable omission: spare strings.

When I was a full-time street performer, and breaking strings regularly, I formed the habit of keeping my previously used strings as spares. My most-recent set normally included only five (or fewer) intact strings, so I was always sure to have a spare replacement for every string by keeping strings from older sets as well.

Nowadays I replace sets of strings before ever breaking any, but still maintain the habit of keeping a used set of strings in the guitar case for emergency replacements. If I have any brand-new strings, they stay at home, where I will install them when I'm good and ready.