1: Don't worry about Mary Sues, because the so-called 'pandemic' is a wee bit overblown...
Contrary to popular opinion, there are several things which Mary Sues are, and several things which they aren't. They are occasionally annoying, they are very much the wish fulfilment fantasies of whoever happens to be writing them, and they do detract a lot from some stories by having an undue amount of focus put onto them because such level of focus subtracts from the development of other characters. That said, they are not automatically bad, they are not decided by an arbitrary list of factors such as hair colour or eye colour or accent or angst-filled background (if it were true, my shifting accent and years of bullying would qualify me), and they are not in any major way supported by academic discourse.
I've studied literature for three years now, and I have only once seen a professional author refer to the concept, and that was in Superheroes!: Capes and Crusaders in Comics and Films by Roz Kaveney. She doesn't put much focus on them, but it's notable that her discourse is easily the least masterful of the books I've used, given that she has a background as more of a reviewer than an academic (it is notable that her work cites very few other academic sources, and does not actually have a very extensive bibliography - which is to say that it does not have one at all).
To be honest, it would be more prudent to actually establish what a Mary Sue is; is it someone with bright multicoloured hair or kaleidoscope eyes as applied arbitrarily to the work in question? Yet Delirium from Sandman qualify on that front, despite quite clearly being one of the most unbalanced yet most interesting characters from the comic in question, and this really is saying something. Is it the inability to be killed via conventional methods or being otherwise immortal? Despite the fact that I can mention at least two comics, two television series and a film where these characters are perfectly suited to the environment and tone; it's part of the whole point that they're so indestructible and yet can still remain interesting (if you're wondering, The Crow, Watchmen, Torchwood - at least in the second season, Captain Scarlet and Highlander are the works in question). Is it that they're angsty? But let's face it... I've got twelve shelves covered in novels, plays, poetry, films and games and most of the really good ones feature angst as part of what makes it good (see spoiler tags below).
Or is it ultimately the amount of undue focus upon the main character? But that would more or less be the point, wouldn't it, of a main character... you could argue that there's an incredibly high amount of focus on what Superman's doing over what Lois Lane is doing, but the comic's actually called Superman (or Action Comics - Superman's one of a few superheroes who has no less than several books dedicated to him at once, along with the Green Lanterns, Batman and X-Men), and not Lois Lane. You could argue this point in fan fiction, but that's sort of what I've been leading up to, as the Mary Sue is a construct of fan fiction; its original form was the super-beautiful junior officer who was half-Vulcan like Spock, could out-doctor McCoy (whilst also being a 'physicist/bricklayer/etc.' which McCoy often claimed he wasn't), and could out-Kirk Kirk whilst winning his affections.
Ultimately the Mary Sue is far more a phenomenon of fan fiction than original fiction, and its definitions are flimsy at best. I could create a non-Mary Sue character using most of the traits on many Mary Sue litmus tests (because they don't work), but similarly I could create a Mary Sue which doesn't even register on them because the whole thing is about fucking context. Well, I probably could. I don't really see the point except poking certain types in the eye, so I wouldn't really try. I will be revisiting this later in more detail as part of more in-depth academic analysis.
2: Try not to write angst unless it's genuine...
This ties in somewhat with a friend of mine's hatred of emotionless mass-produced novels, but really ties in mostly with the point raised about angst... it's actually somewhat annoying in most of the works it appears in. Ostensibly. You see, I don't buy into the fact that angst is annoying, because it sells ridiculously well on certain markets and does in fact have its own virtues in that it's about the evoking of an emotion. And yet what we run into time and time again as part of every single fucking review I read is 'enough of the teenage angst' or 'oh look, more whining'; but this is because of something I've ultimately come to a conclusion about.
Angst doesn't usually work unless it's genuine, or the writer's really, really good, or both... there are many examples, but I'll try to limit myself to a few of them for the purposes of this. In Twilight, we have a seventeen year old girl worrying about the fact that she's getting older and her boyfriend doesn't age. She angsts about the fact that she's not a vampire. She angsts about the fact that he's too awesome for her. She angsts about the fact that he's not there. She angsts about the fact that she's ugly when every man she knows is attracted to her. None of this feels genuine. It's just generating false emotion to elicit false empathy from the audience.
In other terms, however, we have the Crow. It was written by a grieving man who'd lost his fiancée to a drunk driver who was attempting to work through his angst by writing a comic about it; it's actually based around an incident similar to the one which led to the author's grief. A young couple who are very much in love and in an extremely genuine and moving relationship are brutally murdered; the woman, Shelly, is specifically raped and killed, and the man, Eric, is shot and left paralysed so that all he can do is watch as the mutilation of his fiancée unfolds. This part is told in flashback. The major part of the story follows Eric, resurrected as an immortal being in order enact revenge upon the murderers whilst suffering the endless grief of being parted from his love... this is about as happy as it gets. The comic is dark, gothic, brilliant, and awesome... it's also very angsty, and the reason it works is that you really feel the emotions involved.
You get a genuine sense of the pain of unfair loss, and the sort of emotional turmoil that the author and character were going through. It's this sense of reality, and genuine emotion which drives the story forwards for the reader, and it's really very much why angst is popular; when it's done right, the audience also feels something for the character or characters. Incidentally, the fact that the character serves as an obvious author surrogate, thus a 'Mary Sue', and does not detract from the story whatsoever is a bit of food for thought.
That said, really good writing can produce the feeling of genuine angst because a really good writer can make himself or herself feel these emotions.
3: Avoid trying to make your story 'marketable'
This stems from what I've seen of the Superhero Nation website (which I've mentioned to some of you over messenger programmes) and this serves as a rebuttal; one of the major recurring things on that website is its creator does not actually offer all that much advice when it comes to telling a story. He offers much advice when it comes to selling a story, and the major problem with that is that you lose a lot of the real strength of storytelling that you might have been able to have when you try to sell it rather than tell it. You see, as much as you're playing to what's good and safe and marketable, you're losing out on a lot of the soul of the thing. This is exactly why there's very little strength in the Joel Schumacher Batman films versus the Tim Burton or Christopher Nolan films. No matter which of the latter two directors you think captured Batman better (I actually think Batman and Batman Returns were slightly stronger than Batman Begins and The Dark Knight because they captured the feel of the central character a bit better, even though the Joker of TDK was far superior, but it's really like comparing cool and awesome), many people can agree that Joel Schumacher's films just plain suck.
This is mostly because the Tim Burton and Christopher Nolan films attempted to explore the character, and his relationship with other characters, as opposed to selling something (y'know, endless variations on the Batmobile and Batsuit aside), because their primary goal was to tell a story. This proved to be insanely successful, at least in the case of the Nolan films. The Joel Schumacher films, on the other hand, were far less about storytelling and character and far more about drawing in an audience with shallow little attraction notes; 'oh look, this film has Val Kilmer as a starring role, and the sequel has George Clooney and Arnold Schwarzeneggar making ice-related puns!' Hell, they cast someone who everyone originally mocked for the choice of the Joker in The Dark Knight... but rather than being cast for marketability 'because he'd been in a few famous films', the late, great Heath Ledger was cast for performance because he was easily the best person to play the Joker within the framework of the Nolanverse.
This sort of mindset can not only very easily be applied to writing books, but it's arguably more essential. I've never seen a successful or memorable book which was written purely for marketability; even Twilight and Eragon, crappy as they were, are built around the framework of telling the story that the author wanted to tell, as opposed to selling anything. That Twilight sold well was a secondary aspect, as it wasn't created with marketing in mind, it was created as author wish-fulfilment... and though it was crap, I have far more respect for it than the Joel Schumacher Batman films or anything Superhero Nation puts out because, let's face it, there is far more creative integrity in a crappy vampire romance novel written from the heart of an evidently-dissatisfied middle aged Mormon housewife than there is in manufactured stories built to appeal to a lowest common denominator.
Oh, and the one with most creative integrity of the two? It was actually able to produce something more successful financially. Odd, that.
I've got another couple of tips being revised and expanded, along with these three.