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If your inspiration story is in the public domain (which all of Shakespeare is), you have no legal obligation to disclose your source material. Though with Shakespeare, people will of course figure it out.

The musical West Side Story is a well-known retelling of Romeo and Juliet. It was in fact pitched to producers as such and of course all the reviews mention it.

West Side Story is a musical with book by Arthur Laurents, music by
Leonard Bernstein and lyrics by Stephen Sondheim. It was inspired by
William Shakespeare's play Romeo and Juliet. (ref)

A 1961 movie poster doesn't mention R&J though (I can't read the fine print so maybe it's there).

If your inspiration story is not in the public domain, then you need permission from the copyright holders before you proceed. If they grant it, they will advise you on how to cite.

But what if your inspiration story is in the public domain but obscure? I would put it in the acknowledgments, if not the title page.

And if the story is not public domain but your inspiration is slight? That's the trickier one. The answer is "it depends." But err on the side of caution and get permission. Or change your story even more.

I would suggest you read this link (with actual lawyers responding).

Basically if you are not infringing on a copyright, you don't have to say anything. Your example of Romeo and Juliet is in the public domain; but for other works you MIGHT be infringing on a copyright. Here is the part of the 2nd answer at the link which provides detail:

By Pamela Koslyn, intellectual property rights attorney: Copyright infringement is a difficult concept and not easily explained in a forum like Avvo. Basically, courts will look at the total look and feel of two works, and compare how "substantially similar" they are. The entire work is considered, but its unprotectable elements get disregarded. Disregarded elements include themes, ideas, facts, and obligatory to the genre or cliche scenes , called "scenes a faire," such as doomed young lovers, with a scene of angry parents forbidding the love. The original creative elements, such as tone, setting, dialogue, characters, pace, etc. etc. are compared point by point, and if enough of them align, there's infringment, especially if you can also show access to the work. Where access is obvious because, in your example, "Harry Potter" is a famous published work, then the standards for similarity are lower, and where the access is questionable, more similarity is required to find infringement.

Plot points, or "beats" in tv-speak, are among the creative elements that can be infringed, so it's not true that plots aren't part of a what's protected in a work's copyright. [... more material by Ms. Koslyn clipped for brevity ...]

Ms. Koslyn's Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.

As long as you're not committing flatout plagiarism, it's not strictly unethical to write a story that is inspired entirely or in part by another work.

It's no secret that character of Sherlock Holmes was influenced by Edger Allen Poe's C. Auguste Dupin. In fact if you read Scandal in Bohemia, you can see the similarities between Doyle's story and The Purloined Letter, which was published much earlier.

Using your example, Shakespearean works have inspired countless works.
Forbidden Planet - The Tempest.
The Lion King - Hamlet
A major storyline in Gargoyles was based on a reimagining of Macbeth.

No one flatout said The Lion King was inspired by Hamlet. But the story is still there and it's not a stretch to see where the inspiration came from.

As you have selected, it is not necessary to credit a story which is in the public domain. However, I notice that several of your questions have not been answered. I'll address them.

Also, do you mention it in the title that's it's a reimagining, or
would you put it in the preface.

You certainly can mention it either way, even though you don't have to. As an example, if you want to tell the story on a Mars colony in the future, you could title it, "Romeo and Juliet and Olympus Mons", or something similar in order to pique the interest. A well-known and prolific SF author, David Drake, is famous for his retelling of historical battles in a far-future setting, and he has been very forthcoming in his forwards as to exactly when and where the original occurred.

I see it a lot in movies, but sometimes, they don't put any disclaimer
or anything, and sometimes it's in the title and sometimes it's not.
Is there a guideline on how to inform your readers in an ethical way?

Movies and TV are a somewhat different story. The industries are full of lawsuits about misappropriation of stories, and some of them have been quite successful. In general, a movie which actually uses an existing story will negotiate with the owners of the story and pay for the use. In such cases, no attribution is normally made. There are exceptions: "Breaker Morant" and "JFK" both included book credits at the end of the movie. But in general, it is not ethical (or legal) for a movie to make use of a non-public domain story without paying for it, let alone without giving credit.

I would like to suggest something else to consider.

Not the legal requirements, that is already covered off very well. What would your readers like? And even more what would your characters like? Something that could be useful in terms of advancing your material - is to have your characters have a degree of self awareness.

Depending on the nature of the story, working a little awareness of the classic scenes from Shakespeare's Romeo & Juliet into the storyline in an internally consistent way could help things along a little bit. EG is poison or suicide a part of the plot line? Play on the similarity, by having your main characters be Shakespeare fans - or fans of any one of the remakes of that classic tale:

• West Side Story


• William Shakespeare's Romeo + Juliet


• 
  Grease (I personally have always thought this was a Romeo & Juliet story)


• Gnomeo & Juliet


• Warm Bodies

According to this page:
http://www.tcm.com/this-month/article/191916%7C0/Romeo-and-Juliet.html

there are more than 30 versions in the movies. This gives a wealth of material that will be very accessable to your characters, and that they could get a teeny bit obsessive over