Simply write "(c)"Copyright: Your Name. Do not copy." Putting your name on it leaves them a way in in case they want to contact you for possible re-use or collaboration. The "do not copy" (or similar desired "term of use") puts your explicit intentions on the document, in this case reserving all rights to yourself. I personally put the latter part in a different font so that it stands out as a "meta-item" of the document.
Otherwise, if you think the "cat's out of the bag" on your material already, the best way to protect it is to publish it to the web, where the Internet can form a historical record for your creation and the community can now help you with fair credit assignment.