Below is the guidance I propose; it already appears on the tag info page for story-identification since April, but the version below has some minor additions that I did not yet add, now that we have a discussion here.
shamelessly copied adapted from the relevant discussion on Science Fiction and Fantasy (most heavily from Valorum's answer). I think it covers all the basics, or at least everything that can be covered without seeing the actual question.
This tag should be used by people who have read/heard of a story, and can no longer remember the title of that story. People using this tag should provide detailed descriptions of everything they can remember about the story, to help those answering identify it correctly. Please do NOT use this tag to ask for literature recommendations - it is meant to find stories you have forgotten.
A list of details one should add to their question includes, but is not limited to:
A meaningful title
The title, ideally, should mention the key points that would make the people who see the title and have read the story identify it immediately, or make the people who have not read the story become interested in it. Because of that, titles like
Looking for a short story I read as a kid are not helpful at all. A title like Overpopulated world, where volunteers are being taken to be converted to food, on the other hand, seems to have at least some connotations, so it has attracted comments and an answer with guesses.
The age of the story
Was it an old crumbling book or a fresh one? Did you see it in a magazine from the 1980s or on a website yesterday?
Time when the story was read
10 years ago? Last summer? "When I was in high school" is not descriptive (we don't know when you were in high school) - try to provide a year range instead.
Magazine? Short story collection? Was it a novel? Maybe printed in a textbook? Was it an online story? If it was a story found on the Internet, mentioning the websites you used to frequent is a good idea. For print sources, a publisher or a book series may be of help.
Language of the story (i.e. culture of origin)
In case you read the translation, try to provide the language the book was originally written in - this will narrow down the search area significantly.
Language the story was read in
If it was a translated work, provide the language you read it in. This also helps to narrow down the area of search significantly.
Location (where was this story read?)
Public library? Airport book shop? USA? Italy? Any of these will help.
The appearance of the book
What did the cover look like? Some questions on Science Fiction & Fantasy have been answered through the askers describing the appearance of the cover of books or through rough sketches of the work they are describing, such as this one.
What have you ruled out?
What stories is it definitely not?
Every little detail will help - do not omit anything just because you think it is insignificant.
This text was partly adapted from How to ask a good story-ID question? on Science Fiction and Fantasy Stack Exchange Meta.
For examples of good story identification questions, one can look at the story-identification questions with highest scores. While score is not always indicative of quality, I find those questions very well worded, and I think the fact that most of them (and over a half of all story-ID questions) have an accepted answer is indicative of their high quality. As examples, those 3 questions - Short story in Russian about time travel and changing the history of WW2, Sci-Fi Short Story - Life Saving Weight Loss, and Overpopulated world, where volunteers are being taken to be converted to food - are definitely good enough, and even great, as example questions.