Tip #1: Write about what you know. If you're writing a love story in which the main female character is dumped by her boyfriend, think about what you have been through in your own personal experience, and think about how she might react. Does your character have a strong personality? Are they normally quite likeable? Do they have a weak personality, and they let people push them around? Or do they have a personality that is mysterious, and unpredictable? Once you have established a main character, only you, the author, can predict how they will react to a certain problem.
Tip #2: When beginning a story, and a chapter, it often helps to start the story/chapter in the middle of an action, because then you immediately grasp the reader's attention.
Tip #3: When writing a summary, you might want to include a very short excerpt from your story. That way, you get the reader intrigued. In a real, published book, the first thing that a person sees is the cover, second the title, and third, the back of the book, where the summary is usually located. But unless your work is already published (in which case, you probably don't need to read this), you don't have a cover for your stories, and the first thing the reader sees is the title, and then the summary. In order to capture the reader's attention, make your summary brief, and don't give too much away; you want to keep the reader wanting more.
Tip #4: Write about what is important to you. If you're writing about breaking up with a boyfriend, and that is something that has never happened to you, or is not important to you, then you can't write to your full potential because your heart isn't into it.
Tip #5: Your plot always has to be moving and getting somewhere. You don't want your story to hit a stand-still. You need a means of how your characters move forward in your plot, and you, the author, always need to know what route your plot will follow. You don't need to know all the details; just the basic plot line.
Tip#6: In your fictitious world, you need your limitations. Your characters are not invincible, and they can't live forever (unless their elves :-D). They don't breathe fire and they can't create force fields. Although many authors here I'm sure, myself included, wish that writing was that easy, it's not. You need to stick to the limitations that you give yourself (when writing an original story), or the limitations given you (when writing a story on fanfiction). If not, then your story doesn't make any sense. The rules that you created for your world in the beginning is how they MUST stay.
Tip#7: Description. This is a big one. You have to remember that the world you are trying to create through the use of words is one that most of the rest of the population may have trouble picturing, so describe, describe, describe. It doesn't have to be so filled with so much detail that the plot virtually doesn't exist, and all you're doing is setting the scene throughout the whole book - remember, the story must be going somewhere - but detail is still important, and it does set the scene, making it easier for the reader to picture what you as the author are trying to portray through your writing.
Tip#8: This has a little to do with the description thing. When you want to write something, and you struggle with describing stuff, get your ideas down on paper (or computer) first, and then add the description in afterwards. Now, for some people, they can just add it in as they go along. But if you are the kind of person who has so many ideas bouncing around in your head that you can't make heads or tails of it, you need to follow this tip, for your own sake as a writer. You need to make sure that you get your ideas down before you actually write anything substantial. That way, you will already have the ideas down when you start writing, and you don't need to worry about forgetting them.
Tip #9: This one was told to me by my friend of mine on fanfiction, whose penname is OnMyKnees. I learned this from her, because she gave me advice about my story, The Darkest Night. When writing in first person (third person, too, but mostly first person) you want to stick to one character. If you want to put it from another character's point of view, then write in third person, so your audience can relate more and connect more to your main character.
Tip #10: This one and the next were told to me by Direlda, here on dA. First of all, read as many books by as many different authors as you can. Find one or more authors whose style you like, and then try to imitate their way of wording things. Now, some may say this is plagiarism, but it's not. In fact, I myself have had to do exercises in school where I had to imitate an author's style of writing. this kind of exercise is good because it allows you to realize the difference between your own style and the style of your favorite authors. But that doesn't mean that when writing original stories/poetry, you should think, "How would so-and-so write this?" Of course, you have to add some of yourself to the mix. But doing some imitating exercises can not only help you see different ways of describing things, it can also enhance your vocabulary tremendously. The reason I am where I am is because the first novel I ever read was by C.S. Lewis. If anyone has read any of his books, then they know that his style of writing is very old English, and he has an extensive vocabulary used. (There are a lot words he uses that I don't understand; it's good to have a dictionary on hand, as well).
Tip #11: Practice (I can't believe I didn't think of this myself!). This is important. One can never improve if one does not practice. Dont' be discouraged if you think you're writing isn't good enough. It always starts out like that. (you should read some of my work from years ago. It was bad. And when I say bad, I mean bad). As they say, practise makes perfect, and it's absolutely true. If you don't think that you can self-edit enough to improve (which is a part of practice), then ask a few people to be your "editors" - people you can rely upon to edit your writing, find out where they think you can improve, and tell you where your strong suit is.
Tip #12: This is a kind of continuation of the previous tip. Just as you appreciate someone else's criticism, others will appreciate yours (not all, mind you). This is important because - not only will it help them - but you may find out that you are doing some of the same things they are doing that need improvement. Often, it's easier to find fault in someone else's work than with your own, so if you see them doing something that doesn't seem quite right, and you're doing the same thing, it will help you realize where your mistakes are much more readily. (Oh, and try to be polite when giving criticism. So many people don't realize they're being rude until someone gets all PO'd at them.) And, above all, when critiquing someone's work, always find at least one area where you think they are doing well. It tells them their strong suit, and also gives them a chance to improve other areas.
Tip #13: This was suggested to me by ~pralinkova-princezna. You need to have motivation. If you love writing poetry, but hate essays, it's going to be a lot harder for you to write an essay, than it will be to write a poem about the same topic. It has to do with what you're passionate about. Also, as a part of this, you need to be aware of who you're writing for, and why you're writing. If you have no purpose, then there's really no point. Even if you're only purpose is just for the pure pleasure of it, and just because you love to do it, then that's good. But unless you really know why you write, the motivation is less likely to be there.