1. Know your structure.
And, no. I'm not talking about writing an outline. I'm talking about being familiar with the structure of what you write. From the basic beginning, middle, end to more complex sub-plot arcs, character development and so on. Short stories are simpler than novels. Not necessarily easier, but certainly simpler. They contain fewer characters, fewer plot points. A novel is a gargantuan thing that if you start at it willy-nilly is likely to twist and break and crush you under the weight of there's-so-much-here-I-didn't-take-into-account.
2. Know your limitations.
The adrenaline rush of starting a new project tends to make me jump in with both feet. And that's all well and good, but it's important to know one's limitations. Even at my most excited and fastest typing speeds, I can't write a novel in a day. I'm lucky to get a rough draft in a month. This is one reason why goals are important, they tell you where you're going and keep you from burning out by running full steam all the time. In other words, work hard, but don't work too hard.
3. Getting the big stuff down is only the beginning.
A common misunderstanding among newer writers is that the purpose of editing is simply to look for typos and maybe tighten up a few sentences here and there. We've all felt that initial rush of Thank-god-it's-done that comes when we complete a rough draft. But if you're like me, you're also beginning to realize that the rough draft is just the beginning. Then comes cutting out the dead stuff. Killing those darling that just don't fit with the rest. Filling in plot-holes. Smoothing out character arcs. And so on. The rough draft is the easy part and once that's done the real work (of revising and rewriting) begins.