People quickly learn to ignore things that are always there.

One way to deal with this would be to select the wording of the message at random (a la 'fortune'). So, you might forget the code tags on four separate occasions and get a different one of the following each time:

    Code tags in your posts, 
    These are a great goodness, true
    Alas, all unfound

        (or the alternate version of the above:)
  
        Code tags in your posts, 
        These are a great goodness, true
        Unlike this haiku


    Code tags, code tags, wherefore art thou code tags?


    You know...that looks a lot like a code snippet, 
    but nary a code tag in sight.  You sure you want 
    to post?


    I'm sorry <username>, I'm afraid I can't do that.
    At least, not until you add some code tags.