If you happen to be with you best bud and looking over thier shoulder as they surf their favorite forum, help them pay attention to the date and time stamp of the last post in a thread. If the post is really old, don't let them reply to it. Thread Necro can lead not only to the embarrassment and poking fun of your friend, but to that of others who now think that thread is new and relevant since it has been bumped to the top of the forum.
Exceptions can be made if they are the Thread Owner. Thread Owners can resurrect their own Threads should they have some sort of update that people might have been expecting.
The Public Service Announcement was brought to you by the Thread Paladins Alliance; Fighting Thread Necromancers since 2007.