The summary is hard. I know that, if from nothing else, than from the plethora of stories whose summaries consist of "I can't write a summary plz r&r anyway". So here's a few guidelines that may help.
Your basic goal, when you write a summary, is to try and convince people to read your story. You need something that sounds creative and original, and that intrigues people so they decide to click the link.
Try thinking about your story for a minute. What's its tone, what's its mood? Is it angsty or happy or a thriller? When you write your summary, keep in mind how your story reads. You won't want something long and rambling for a short, direct one-shot; similarly, you don't want something that's really sparse for a sprawling epic.
For example, from one of my own stories:
I always let you go 'cause you got a pretty face, Cowboy. But it ain't gonna be pretty no more when I get through with it. [rated for slash and general creepiness]
What does that tell you about the story? It's in the first person, and it's about someone who wants to hurt Jack. The bracketed note at the end explains why it's PG-13 and not just PG, which is what I generally rate slash; it's got some other elements to it. (Also frequently labeled are stories with language and violence, so the reader knows what to expect and the chances of a story getting deleted are loweredFFnet seems to be cracking down on story ratings lately.) But the tone of the summary matches the story itself.
Furthermore, it leaves the reader asking questions about the storywho's speaking, what does he want to do with Jack, and why? To find out those answers, the reader clicks the link.
It's hard to explain just what to put in a summary. When in doubt, fall back on a good old honest description:
They've had a fight, and now Blink's best friend is gone, maybe forever. Deals with Blink's past and his relationship with Mush.
Certainly not the best summary in the world, but it gets the job done. It tells you what the story is about; if you like Blink or Mush as characters you'll know to look at it. If you want to know what you're fighting about, you'll look at it. The summary isn't exotic, but it works.
However, it's much easier to list off things to absolutely avoid putting in your summary. Here goes nothing...
* Don't say "I can't write a summary" or "I know this summary sucks" or "summary inside." As always, I can't speak for anyone else, but I'm not going to look at any of those stories. If you admit you can't write a summary, why should I assume you can write a story? I have too short of an attention span to go looking for it; there's a reason the summary is under the title on the main page.
* Don't say "this is my first fanfic," even if it is. There's no need to advertise if it's your first story; a good story will be good anyway, a bad story will be bad anyway. If you have to say it somewhere then put it in an Author's Note. But labeling something as your first fanfic implies you're not sure it's good, and if you don't think it's good, then why should I?
* Don't ever, ever, ever say (in summary or AN) "I know this sucks so just r&r" or anything like that. Don't ever own up to your work sucking. For one thing, don't post things that suck; save us all the time and energy. If you know it's bad, make it better. (Read through the guide again. That's what it's for.) Pretty much, what that means is you're saying, "This is bad, but don't tell me it's bad, because I don't care enough to improve it." And again, if you don't care as the author, why should I, the reader, take the time to care?
* Don't demand feedback or threaten to discontinue the story if you don't get it. People are reading the story because they want to, not because they're obligated to; ditto for reviewing it. No one gets as many reviews as they want, ever. Because it's not anyone's job to review. Think of reviews as presents; they're great to get, but you should be grateful for them.
As for threatening to stop posting the story unless you get X number of reviews, well, go ahead. Stop posting. Then you don't get any reviews, and prove yourself to be pretty immature.
You should be writing because you love writing, or at least because you love the movie. If you're writing just for people to tell you how good you are, find a new hobby. If you're writing for instant gratification, find a new hobby. If you're writing so that you can hang out with the newsies... Well... Maybe keep it offline. ;)
Another word of caution: beware of The Question. Yes, it's good to have the reader asking questions and wondering what happens, but a lot of summaries use The Question and it doesn't always work.
What is The Question? It's a simple formula, where you ask a question that your story answers. If I was to rewrite the first example summary I used in The Question format, it would probably read, "What happens when Oscar wants to hurt Jack because he's in love with him?"
I shudder at that. Aside from the fact that it's grammatically awful and awkward (those he's and hims should be clarified so you know who's in love with who,) it also gives out one of the more major spoilers of the storywho's stalking Jackbefore the story even begins. Be very careful if you're writing your summary as a question; it's hard to do it and have the summary come off as mature and interesting.
Even more to the point, if your story features an OC in a prominent role, be sure the summary focuses on a canon character. Why? Because most people are reading fanfiction because they like the fanfic characters, and don't know or care who your OC is.
"What happens when Elsie gets to NY and finds out her old enemy Spot is there?" Who cares? Who the hell is Elsie?
To explain how to improve, first, Elsie is my guilty pleasure SI. Someday, that story may see the light of day, but not in that form. But if I ever do write it, the actual summary would be closer to, "There's nothing Spot Conlon can't handle, at least until he meets the girl who killed his brother." Because that places the emphasis on Spot, who we do care about, not Elsie, who we've never heard of.
Again, to speak strictly for myself: if the summary focuses on an OC, especially if it's a romance, and especially-specially if it doesn't even mention a canon character, I'm not going to bother.
Furthermore, even for a character who's recurring in your own newsieverse but not canon, you should probably still focus the summary on the canon character, not your OC. New readers won't know who they are, and frankly a lot of people don't care and just don't have the energy to find out, unless the summary inspires them to.
One final note about summaries: spelling, grammar and punctuation are just as important here as they are in the story itself, if not more so. If you screw up a few times in the story, that's forgivable, but in the summary where you're advertising your fic? Forget it. If you can't spell your own title or place a freaking comma, that doesn't exactly inspire faith in the story itself.
Internet slang (plz, ppl, u instead of you, UR instead of you're or your, 2 for to, too or two, etc.) is never, ever, ever acceptable. This is another of those non-negotiable things. Just say no.