Good advice here.

Be methodical.

Be consistent.

Keep your certs and medical on your badge lanyard. Makes it hard to forget that stuff.

Buy professional luggage (suitcase + toiletries kit, and probably a modular flight bag with cooler and laptop/notebook storage). A standard 22" unit will work fine for a four day, and if you really cram you could go six. Since we only have max four day trips, if I get a back-to-back trips of more than four days total I'll stash clean clothes at my base and swap them out between trips.

I use a freezer bag for dirty gym clothes, but just roll up socks/undies in used t-shirt. As other have said, you need clean t-shirt, socks, skivvies daily, white shirt every other day.

I rinse gym clothes in the sink if I'm going to use them more than once. Wring out, roll them tightly in a towel to remove excess water, and then hang dry. May need to hang near heat/AC vent if it's humid.

I only carry running shoes, those double as civvy shoes if I go out. Some people also bring flip flops.

Carry a light civilian jacket. If you need a serious jacket in civvies, take the bling off the airline jacket, which is presumably suitable for whatever climate you're in.

Have a personal checklist, mentally run it EVERY time you stand up, exit an airplane, hotel room, van, restaurant, etc.

Mine goes like this: Wallet, Badge, Cell Phone, Glasses, Jacket, Laptop, EFB/Flight bag, Kindle. I use this all the time, even when not at work, keeps me from leaving my wallet or jacket in various places. It becomes second nature after a while, and requires no effort (but saves plenty of hassles).

I don't include cheap stuff (like chapstick, comb) in the checklist, keeps it minimal and focused on important stuff. So I lose combs sometimes (but I carry extras in the luggage)

Don't set important things down in the plane. I leave snacks and magazines in the plane all the time, because I set them down and don't see them in the dark. But I never leave wallet, sunglasses, cell phone in the plane.