Always carry a pen, a USB memory stick, a pocket umbrella, a bottle opener, a condom (some of these examples may not apply to you) etc., as you may find yourself needing one without knowing beforehand.
I do not regularly carry any of the items on this list and have not felt the need to begin carrying them.
My default set of things to carry with me consists of three items: a wallet, a phone, and a set of keys.
a bottle opener
Swiss Army Knife including a bottle opener and other things
Not that I ever used it, but it feels good to be a 0.1% Batman.
Repositories are awesome, and we should have more of them.
I recently completed BJ Fogg's Tiny Habits, and it's a pretty lightweight way to install new habits (recommended). However, I realized I could use a better repository of useful habits.
So, please use this thread to suggest habits that you've found useful. Bonus points for evidence/anecdata of usefulness.
Obviously, 1 habit per comment makes upvotes a clearer signal of collective approval.