Hi, welcome to the first ever post of Noveling 101! Check out the official introduction and the Tumblr page.
A common problem with noveling is not finishing the novel. I have seven unfinished novels lost in cyberspace myself. So how do we stop that brilliant idea in our heads from dying? The key, I think, is to let yourself continually discover new things in your novel.
When I started writing my first completed novel, I had three things: a list of characters, the idea of the first chapter, and what each character's fate would be at the end. As I progressed through each chapter, I would discover something new: this character is sassy, that character uses a lot of adverbs, this could lead to that. It was a journey of exploration, and I didn't know everything there was to know about my novel until I finished the last word of the epilogue.
So if you constantly suffer from not finishing your novels, don't set everything in stone. A character chatroom, like this one, is great for experimentation. If your characters refuse to go along with your plot (happens so often, sigh), let them. See how it turns out. After all, they (sometimes) know what they're doing. Don't write chapter-by-chapter outlines; draw rough flowcharts to show the relationship between events, and remember to use pencil.
Of course, not deciding everything beforehand can lead to plot holes and logic fails, but hey, you can always edit it. If you lose interest halfway through, it'll end up buried underneath cyberdust.
Another easy way to not finish a novel is by writing the ending first. It works for some people, but whenever I do that, I no longer have the motivation to write the middle bits. Once again, that's because I already know my novel too well; what's the point if the ending's done already? Even if, like in my A Song of Steel, you've decided on the ending already, let it simmer in your head. (Scribble a line in your notebook to avoid forgetting it.) As you write the rising action and the climax and the falling action and all that, things may have deviated from your original plan so much that your ending may need to be tweaked.
Sometimes, the novel comes to me in a smattering of scenes, like bits of a movie. This is both good and bad. Good because as these scenes replay in your head, you'll become excited about writing them; bad because you have no idea how to connect those scenes. My personal method is to, again, let them simmer in my head, and consider them again when the glue or the scaffolding is written. Another way is by writing them all out as possibilities, but in this case you have to be ready to throw it out the window.
(On a side note, one of my planned posts is about how to get from Plot Point A to Plot Point B. Anyone interested?)
One last thing: I've emphasized a lot on not deciding everything, but for heavens' sake don't go in without any plan whatsoever. It's just as easy to lose interest after writing two chapters of pure description/setting/character and not having a direction. Decide on vaguely what's going to happen. Your MC will stumble into a magical world. Fairies will abduct your MC. Donkeys will rule supreme over humans. Something like that.
Hope this was helpful — let me know in the comments! ;)