Early in my adult life I realized that when I cannot find something of importance it is very stressful for me. I became a very organized person because of that. I know that not everyone is or even can be well organized, because for some people it just doesn't "take", so it is important not to feel you lack anything if you cannot get organized in your home. But, a few tips might still be helpful.
1. If it is something important, and there's only one, it has to have its own special place where it always lives. It doesn't matter where that is, just has to be the place where you know it is there. So where should that be? Play a little game with yourself, and imagine that you suddenly need that object extremely badly and you have to start looking for it. In this little imagination, where is the first place you look? that is where you should keep it. It doesn't have to make sense. If that means you keep your passport in the kitchen drawer with the towels, fine, keep it there.
2. If it is a multiple something, have them all over, like JayCs's pens and pencils.
3. If you have looked for something and not found it (Jay's tips on seeking are great) then stop thinking about it. sometimes where you used it last will surface in your mind and it will be there.
4. Write down where you decided to put things in a notebook you keep in only one place and never move from there. You can look at it to find out where you decided to put things, but not take it out of the drawer or wherever you have it - the rule is it has to stay there so it can't get lost. This is especially useful if you decide to change where you are keeping something (although unless it is necessary, I don't really advice changing)