There's no need to prepare yourself years in advance for possible contact with your future self. Some have suggested devising a password that you never tell anybody. If the visitor from the future knows your password, then they're you. Since you don't remember undergoing any time travel, it must happen in the future, so they must be a future version of you, with all the information you have and more, so it would be reasonable to take their advice. In fact they don't have to be you, they could be someone your future self trusted enough to give his password to. Either way, you should trust them, since, in the future, you will trust them.
This is an excellent plan, and it seems reasonable to plan ahead even for a contingency as unlikely as this one. But it just isn't necessary. If someone approaches you, claiming to be from the future and having extremely important instructions for you, just make up a password on the spot, commit it to memory, and then ask them what it is. Your password will have propagated into the future instantaneously, and they'll know it.
Some have pointed out that this scheme can be foiled by telepathy. This is absolutely true, but a minor modification fixes it. First, ask the visitor to write down your password and place it in a sealed envelope. Once you have the envelope in hand, but before opening it, think of a password and make sure you'll remember it for the rest of your life. Open the envelope and see if they match. It's that simple.