If you're traveling abroad, it's okay to forget your toothbrush. Just buy one at the airport or wherever you arrive. It's okay to forget your MP3 charger, you'll survive without music.
But a possession you absolutely must have at all times is your passport. I renewed mine just three years ago, but the last time I needed to use a passport to leave the country was when I was 9 years old!
My passport is one of those things that always seem to be around. Every time it pops up, I'm haunted at the fact that it's in plain eyesight or left in a purse that could be stolen. So, I did the dumb/smart thing a typical Jenny-Woo would do: I hid my passport.

Now time comes around when I finally need it and it's nowhere to be seen! I booked my trip for Europe, there's no turning back. But I can't leave without my passport! Sure, I can pay $100 or more to ensure that I get a rush copy sent to me before I leave. But my budget already tight for Europe, I had to do a deep search in my apartment for my passport that I so cleverly hid.
I did find it. It took two days of ransacking my apartment and looking in the same purses/drawers/cabinets/boxes/corners/under-tables over and over and over again. I even looked under my couch cushions, between my mattress and box spring, behind my dresser, and under the floor mats in my car!
So relieved to have it. Embarrassingly enough, I did Google how to find lost items (maybe deep down, the part of me that highly invests miraculous faith in Google believed that Google would actually say, Your Passport is located in ...). But I did find some good tips, actually. And have some of my own. So, if you are looking for that must-have like a passport, here are tips to follow in order:
1) Look first in plain sight, where it's most obvious. On top of tables, on shelves, on the couch, etc.
2) If you can't find it in plain sight, then look in just the places where you'd think you would have left the item. Check your self (I did once look for glasses that I was already wearing), check your coat pockets, pants that you recently wore, etc.
3) If the item is still at a loss, then look around the usual areas it would be to see if it had fallen behind or under something. Start looking deeper, retrace your steps, think of when you last had the item.
4) Don't tire yourself out. Sit down if you feel like you're not getting anywhere, and meditate. Imagine that the item is there somewhere and you just need to find where "there" is.
5) If you are looking room by room, be sure to look very thoroughly in each place that way you feel certain of where the item WON'T be, narrowing down the possible areas of where it could be.
6) If you feel like you have searched everywhere, don't stress and lose sleep over it. In fact, get some sleep. After a day of searching all over my apartment and car, I let myself forget about it for one night and start again the next day. Having a clearer mind and more energy helped me think of places I haven't looked in.
7) An important thing to remember is to not get stressed and too worked up in your search. Remind yourself that everything is replaceable. Even something like a US Passport you can replace in 24 hours. Won't be cheap, but it's possible.