An author can’t lost his copyright through lack of enforcement. That is to say, you will always be the legal author of the work whether you sue people for copyright infringement or not.

However, failure to enforce copyright can dilute the value of the right. For example, if people can just get your work anywhere, they have less reason to come directly to the author.

However, it is not true that failure to enforce copyright places your work in the public domain. This is a common misperception.

However, you CAN lose your trademark from lack of enforcement. If I fail to assert my legal trademark rights, the law can then decide that I have abandoned my trademark.

Trademark and copyright are not the same. Copyright denotes authorship, while trademark denotes the source of commercial goods.

If you fail to enforce your legal claim to protect the source of your commercial goods, then you no longer have sole claim to that trademark.

Consider the way words like Kleenex and Xerox have become such a common part of the language. Eventually, were the companies that own these marks to fail to enforce that use, they would lose the trademark on these words.

Trademark is an expensive right to assert. Trademark is also expensive to get.

If I choose not to go after every fan who writes a fanfic tale on a blog (and how could I possibly, with my huge legal and financial resources? Seriously…) I lose no rights to my copyright.

However, if fans started making t-shirts and so on, using my characters, then I would have no choice but to shut them down because they jeapordize my trademark. It could also be argued that fanzines using my trademarked title and character names jeapordize my trademarks.

I think a perfect example of your question in action is the case of JK Rowling and the Harry Potter Lexicon. Rowling had no problem with the website, which was a fanlisting and online Harry Potter encyclopedia.

However, the fans got too big for their britches, and decided to publish the website material, which made very heavy use of JK Rowling’s works. Rowling sued to stop it, and won.

That she had allowed the site to remain up and running for years had no bearing on the court’s decision.

If I give a fan a verbal license to write fanfic on their website, consider that at any time, I may also decide to take that license away if a fan abuses the privilege. I lose nothing by allowing fans to write fanfic, but can lose big if I allow them to stray into commercial use and abuse my rights.

The best thing for fans to keep in mind is simply to consider how they would like it if someone did the same thing to their work.