Well I think I understand how to send a message but I'm hesitant to try and send myself a transaction / payment so I will wait until I know what the outcome will be
Open your Bitcoin client, go to the "Sign Message" option (File -> Sign Message in Bitcoin-qt), add the address from your signature in the address field, type out a message saying something like "I <name> control the address <address>" and click "Sign Message". Copy the message and the signature that appears below it into a post.
If you control the Bitcoin address in your signature, this process will provide verifiable proof.