Be Prepared - it's a song in The Lion King, the motto of the Girl Scouts, a popular safety mantra - and, it's one of the MOST IMPORTANT tenants of stage management!
We all know the benefits of thinking ahead - carefully planning out your routes, expertly packing for any surprise situation, and so on - but oftentimes, time gets in the way. A busy schedule can cause people to forget or forego even the most important necessities (phone, keys, wallet), putting them in a rough spot. That's where you, the awesome stage manager, comes in!
When you're stage managing a show, you'll have your entire kit, so you'll be able to supply most of the room with whatever they need (top requests are usually pencils, band-aids, and Advil).
BUT - when you don't have your kit, you should still strive to carry some basic, small items like a pen, a pencil, a Sharpie, a pack of wet wipes, hand sanitizer, a few travel Advil/Tylenol packets, a safety pin, a couple bobby pins, and phone charger. Then, when someone needs something, whether it be at an interview or during drinks at a bar, you'll be the person to save the day. You'll come across as reliable, forward-thinking, and considerate - all great qualities of someone that gets jobs!
So - go forth - and have a pen!