I worked with a couple of older geologists and managers who were fond of standing around and dispensing nuggets of wisdom instead of getting dirty and you, know, helping out with whatever I was struggling with. One of those was "never leave in a hurry", which was delivered by someone who had wandered outside his office to watch my somewhat frantic field preparations.
And he was right, annoying as he was at the time. I had a tendency to get frazzled when I was trying to pull things together for fieldwork, which was usually a couple hours away and involved corraling various people, specialized equipment, several coolers of supplies and piles of paperwork. I often forgot something if I made an equipment requisition list or if I left the office in a rush.
Now, every time I go out, I take a deep breath and go through a mental list of everything I need before getting in the truck. If I have someone else with me (although I usually didn't when I was doing my thesis fieldwork), I'll go through it out loud so that they might catch something I forgot. And that moment of calm is usually enough for me to remember whatever I've forgotten. Failing that, well, I always brought my toolkit to jerry rig whatever I didn't have.