As a (slightly addicted) fanfiction reader, I have spent enough time on fanfic websites to know the difference between good fanfic and bad fanfic. Based on the summary alone, I can usually avoid reading some of the worst stories, but sometimes this works against me. What I mean is, sometimes GREAT stories can be hidden under crap summaries.
I know you can't judge a book by it's cover, but readers usually judge a story by its summary. Here are a few tips on writing summaries that will attract readers (all of the example summaries will be from Supernatural):
1. There is no need to go into the entire plot or your story. Try something simple like putting in a funny/interesting line from the story.
Instead of saying, "Castiel went to a bakery in order to further study these "pie" things Dean was always talking about," you can say something like, "Castiel, Angel of the Lord, finally understood deliciousness." While the first summary is fine, the second is more likely to draw in readers. Think of it like a cliffhanger; you want the readers wanting more.
2. Warnings are necessary, but not for everything. Too many times, I've read summaries that looked like this, "Dean and Sam hunt a vampire. NOT SLASH!!!!"
...Seriously? Yes, slash is certainly something you should warn readers about (some people just aren't into it), but readers don't automatically assume that there is slash just because there are too male charaters interacting.
However, if your summary is something like, "Dean and Sam get stuck in an elevator and are forced to confront their feelings about each other," you might want to put up the "NOT SLASH!" waring on that one. (FYI If you put the phrase "NOT SLASH" in your summary, your story will pop up every time someone punches slash into the search box.)
3. You don't have to list every pairing in your story. Yes, it's standard practice to list the main pairing of your fic in the summary. That is something you should do. However, you should not come up with something like, "Team Free Will celebrates the end of the apocalypse. Sam/OC, Dean/Lisa, Bobby/Crowley, OC/OC, Castiel/Anna."
First of all, never list "OC/OC." Yes, I've seen it before, and it's hardly necessary. The purpose of declaring pairings is to warn/attract readers who have strong feelings about said pairing. If you're pairing OC's, we don't know enough about the characters to feel strongly either way.
Second, only list the primary pairing. If the bulk of the story is Dean/Lisa, that's the only pairing you need to list. An exception to this rule is with slash. Even if your story is 90% Dean/Lisa, you've got to notify your readers if you throw in even a little Sam/Gabriel or whatever else. Sorry, I know it's not fair, but at least you've got something to fall back on if someone flames your story.
4. Be confident, but not a douche. There are two types of summaries that immediately turn me off of a story.
Type A (Self-Hating): "Dean can't cope with the pressures of protecting his younger brother. Crap summary, I know. Please read anyway!"
There is no need to include that in your summary. It's just a turn off to a potential readers. The worst part is, half the time, the summary isn't that bad - minus that plea for readers and the self-deprication.
Type B (Beligerant): "Five times Castiel's head tilt got him in trouble, and one time it got him laid. Slash. Don't like, don't read. Sorry for all of you who don't appreciate the man love!!!!!!!!!!!!!!!"
Just so you know, I'm all for sassy slash fans. You girls (and guys?) rock, but this kind of commentary is better left for an author's note, not a summary. On a side note, take summaries seriously. Abundant exclamation marks and emoticons will only imply that you aren't serious enough to write a good story.
5. It's called grammar. Use it. I know this sounds wrong, but I will not read your story if you cannot manage proper grammar in your summary. If the rules of grammar illude you in a two sentence summary, I will assume that your 1,000+ word story is no better.
This means: capitalize proper nouns and the beginning of sentences, use commas where appropriate, and write in complete sentences. When I see something like this, "dean new their wasnt a monster in closet but he help sam anywayy," I immediately skip over it. Regardless of whether or not it sounds interesting, I'm not interested in reading a story if I have to decipher bad grammar.
One typo is usually fine, but take the time to read over the summary before posting.
I may add more tips as I come across more common mistakes in summarizing. If anyone has anything they'd like me to add (or disagrees with a particular point), just message me and I'll make the necessary changes.
Also, I'd like to expand this be providing actual summaries for stories, both good and bad, that I can use as examples and crituque. If you have one to provide, I'd really appreciate it.