I wrote to myself once (not using the email) http://www.generalforum.com/everyday-life/surpise-49224.html and someone else was in charge of keeping that and they mailed it to me later. I've completely forgotten about it. The whole surprise effect is very important because it also takes you back to what you were thinking that time, which isn't usually similar to current mind frame. Makes you think, really.
Now, unless you found a group or a friend you can rely to do that for you, keeping the letter in a journal may not be as effective. The idea is not to remember it all the time. If you see it (or your keeper) regularly, you'd be reminded of its content. By the time you open it, no more surprise. Now if you really hide it so it's easier to forget about it, after x number of years, you might not even remember it all and you'd never open it.:lol: