Writing is a skill, like playing an instrument or running. And with skills like that, the best way to get better is to practice every day, or as close to it as you can manage. You don't notice much difference from day to day, and some days you have to force yourself to do it, but after a while you get better. You realize that hey, you can write that one scene over and do it better, run faster and farther, play that piece without messing up.
But that only helps if you set reasonable goals. You wouldn't get off of the couch after not exercising since you were forced to in gym class and decide to go run a marathon, would you? If you did, you wouldn't fare well. You'd get hurt, and hate it and never want to exercise again. It's the same thing with writing. You can't just sit down at your computer and decide you're going to write a novel in a week, because that's setting yourself up for failure. When you're just starting out and don't have a routine in place, what you need to do is start one: a word count goal that you can hit almost every day. Commit yourself to reaching that goal every day however you can. Some people just need the goal. Other people need rewards for when they succeed: a snack or soda if they succeed for the day, something larger if they make it a week or a month without skipping a day.
So what's a reasonable goal? Everybody's different. Some people have all the time they need. Others work two jobs and have kids, so they have very little time to write. Writing speeds vary- some people have trouble writing 300 words per hour, while others don't have a problem writing 800 per hour. If you're writing to make money, you'll probably want to set a higher goal for yourself. But as far as popular goals, NaNoWriMo has a goal of 1667 words per day. Inkygirl's challenge has goals of 250, 500, and 1,000 words per day. None of these are very large, but they add up. Even at 250 words/day, you can write most of a novel in one year assuming you only work on one project.
If you choose one of the larger goals and/or tend to write short stories rather than novels, you may quickly find yourself running out of ideas. Then what? If you run out of ideas, you can't write anything, right? Wrong. Some people have more plotbunnies than they want or need, and put them up on the internet. And have you really finished everything you've ever started? Really? Have you rewritten everything to your standards? How about different POVs of some of what you've finished? What happens in 10 years? 20 years? Before the story began? How is [random background character] affected by it? If he doesn't care about it, what does he care about? Everybody in our stories has a story of their own, and who knows- by exploring that, you might find that their story is more interesting than the original. But if you don't write, you'll never find out.
NaNoWriMo forum thread 1, 2 (these links will break when the forums get taken down and reset) (also, adoptable stuff)
Always looking for more plotbunny resources!