Here's how you clearly put something in within the law: (1) You declare it public domain. (2) Now, keeping it there: You simply exercise a level of ethics even a 5 year old understands: You don't go back on your word, because (for one thing) that would make you a major fucktarded scumbag.
That doesn't work. Maybe a year later you get sued for something and the court orders that your copyright be transferred to the person suing you as compensation. Or you go bankrupt and your copyright is sold to your creditors. Or, instead of being sued, you die and the copyright goes to your heirs. And the new owner doesn't consider himself bound by your word.
Furthermore, even if none of that happens, it's still not equivalent to public domain because even if you keep your word, someone who wants to use your work has no way to read your mind and know that you're the kind of person who keeps his word. So he has to act as if you could withdraw permission at any time even if you never would.