Avoiding the Problem
Think about the last thing you lost. Where does it belong? Equipment, supplies and reference material all belong somewhere. That spot, wherever it is, is the easiest place to find whatever you're looking for. Take a moment to put it back where it belongs if you can. If you can't, put it in an inbox.
In his book Getting Things Done, David Allen discussed many reasons for using an inbox. He points out that if you try to avoid an inbox your house, your entire house becomes an inbox. If that describes your house and your life, no wonder stuff gets lost. Create an inbox, or at least an in area, and put stuff there if you can't put it away properly.
To avoid locking yourself out of your house or car, make a habit of having keys in your hand when closing the door. Lock the door with the key, or with the remote. Don't lock the door and push it closed unless the key us in your hand.
This Jedi Mind Trick can help you remember where you put something as you put it down. People learn things through visual, auditory or kinesthetic means, each according to his or her gifts. Which ever you prefer, your memory works best when you create links all three ways. Watch yourself put those keys down. Feel them as they leave your hand. Say to yourself "the car keys are on the bathroom counter under the toothbrushes" and be specific as you do it. In the best case, your learning processes have built strong links. In the worst case, you were paying attention at the time.
Okay, But My Keys Are Lost NOW!
You looked where they belong, where you thought they were, and the floor under where you thought they were. Either they weren't in the inbox or the inbox idea is looking better -- for later. For now, some serious searching is in order.
Picture the item in your hand. Where were you and what were you doing the last time you saw it? Who else was there? Retrace your route, looking most carefully at the last place you were and where you had them last. Is there someone you can talk to?
If that hasn't helped, it's time to make a list. Where have they shown up when lost the past? Where could they be? Who might have seen them? Could someone have moved them? Are there other ways you can look, other people you can ask?
In the end, of course, things do get lost That's why we have hide-a-keys, spare pens, and contingency plans. If life isn't about things, and of course it isn't, it really shouldn't be about missing things.
Develop the habit of limiting the places you put things. Pay attention when putting things down. Visualization can help as you put something down or as you realize it's missing. If you have to search, have a search plan, and always have a backup plan.