No matter who you are, what your story is, or what fandom you are writing a fanfiction for, there will be a time when you are on the front page (as the most recently updated fanfic, if nothing else), and you should have two things going for you that will grab your reader's attention.  Those two things are your title, and the first 300-400 characters of your description – more or less eighty to ninety words.

The title is in a bigger font than most of the rest of the text on the page, and also in blue, so it kind of stands out a bit.  It's basically your way to grab somebody's attention and convince them to take a look at what they can see of the description.

TL;DR

Avoid: clichés, common titles, long titles, question titles, sentence titles, and titles that give away your entire story

Also Avoid: mispelling your title, using words absolutely nobody is going to understand (or be able to pronounce), trying too hard (and avoid calling the title a tittle – that's like calling yourself a writter)

Go for: something that encapsulates the essence of your story without giving the entire plot away, something that sounds catchy and trips off the tongue (e.g. with the help of alliteration, or "[first name] [last name] and the [whatever it is]"), something four/five words or under (four is apparently considered the optimum)

VL;RA (Very Long; Read Anyway)

or

Reasons for the Above

If you are writing in English, then chances are you want to conform to English convention when deciding on your title.  It may be different in your language or culture – in my experience of Korean dramas, for example, there are a fair number (of quite well known ones, too) which have titles for sentences, like My Girlfriend is a Gumiho, and It's Okay, That's Love.  But if you're writing in English, then your communication platform is English and the discourse surrounding English language and conventions.  While not everybody in your reader base will speak English as a first language, they subscribe to that language with some knowledge of how that language functions culturally through conscious or unconscious comparison to their own.  Some will be more familiar with it than others; some will speak English as their first (or sometimes, only) language.  Just like different countries have different notions and standards of things like beauty, the English-speaking world has its own notions and standards of what constitutes a good title from the perspective of an English-speaking audience.  Titles here tend either to be short and snappy, four words long, or longer but with a bounce and a ring to them.  Consequently, you do not want:

1) A boring title, or a title that is a cliché (e.g. His Mate – why does this stand out from anything else?  Mention of best friends and bad boys can usually be classified as clichés when it comes to titles too)

2) A title common to a lot of other works (e.g. Kingka's Mate – why does this stand out from anything else that's similar and make me want to read it?)

3) A sentence (e.g. The Kingka Is My Mate! – this is a statement, not a title)

4) A question (e.g. The Kingka Is My Mate? – wow, shock horror), particularly if it ends with an interrobang, like so: The Kingka is My Mate?! (double shock horror)

5) A title longer than four words (e.g. My Mate Is The Kingka – too long, attention gone)

6) A title that basically gives away your entire plot (e.g. The Kingka, My Mate – if I saw this title, I would immediately assume 1. it's a high school au, 2. the guy and the girl won't initially get on, 3. he's a werewolf (or fate intervenes), and 4. they're going to fall in love anyway.  Oh, and there will probably be a love triangle.  And the plot will centre around these.  And it's not really a plot I can immediately differentiate from dozens of other similar ones.)

Note: I don't know if any of these are actual titles: I just made them up.  I apologise if they are.

Under most circumstances, that is.  Obviously, there are people who can – and do – break those guidelines.  Off the top of my head, though, there aren't many famous ones that break the mould.  If you're going to break the mould, then you might at least do it well.  I've seen it done well in some cases.  But most of the time, I haven't.

When it comes to 1) clichés, the famous ones are the ones from which the cliché derives (e.g. Beauty and the Beast), so weren't clichés the first time they were used.  There's a handy list of what can be considered clichéd titles here.  As for 2), titles which are shared with multiple other stories are just a pain, particularly if they're generic.  Call your story something like, say, Loving My Best Friend, Forever, Elsewhere, The Chosen One, Possession, and then keying it into a search engine will probably give you more than one result and that, for somebody searching for your story is... well, irritating.  And confusing.  Especially if they don't know which author's version they're supposed to be reading, and even more so if the description doesn't tell very much... or if the plots are relatively similar and the descriptions are consequently similar.  Specific to AFF for both of these, including "annoying" or "husband" (or even "jerk") is likely (not certain, but likely) to land you on the bad side of these.  Especially if it's in any way combined with a questionmark or informal interrobang.

Referring to 3) there are successful titles that are sentences, but not very many.  Also, the only two I can think of off the top of my head are Some Like It Hot (great film, by the way) and One Flew Over The Cuckoo's Nest (fabulous play by Miller which I have seen numerous times).  Sporcle has a quiz listing more (for films).  One thing you'll notice about both the titles I've mentioned, though, is that, while they both encapsulate the essence of what the work is about, they don't give things away.  Some Like It Hot involves two guys (criminals?) crossdressing to escape a notorious ganglord and is a comedy.  The title indicates comedy and also that things are going to get pretty tense in some places.  It wouldn't be nearly so interesting if the title was We're Crossdressing To Escape The Mafia Boss.  Like, hello, plot tension?  Oh, oh, you've gone on holiday?  Oh, right.  Bye, then.

One Flew Over The Cuckoo's Nest is about patients in an asylum.  The cuckoo's nest is the asylum, and the one who flies over the nest is McMurphy.  But it's not something you're going to get without knowing the story, and even then you might not if you miss the nursery rhyme quoted in part 4 of the play.  Again, I Want To Escape The Asylum doesn't have quite the same ring, right?

As for 4) title questions, I can't think of any famous titles off the top of my head (yes, I know there will be some) other than Where's Wally?  Which is a kids' book.  Like a large number of book titles that are questions.  Or something like Why Is Water Wet? And A Thousand Other Things You Always Wanted To Know.  Sure, there are great ones out there, like Terry Pratchett's Where's My Cow? (but this is the Discworld we're talki