A friend asked for some extemporaneous writing advice, and this is what I came up with:
Just off the top of my head, I think the thing that is most important about it is not letting yourself get sidetracked. If you need to research something, make a note to yourself and research it later. If you’re not sure about your phrasing, note it and keep going. If you’re not feeling inspiration, well, that sucks, but keep going. If you think you’re not actually any good, keep writing, and you will get better.
If you get an idea that is fabulous but would require a ton of revision to work, make a note of it, pretend you’ve done it, and keep going. You can do the actual revision later. Don’t re-read the old stuff unless you’re just briefly checking something, don’t edit phrasing or correct typos (often a waste of time since you’ll be editing a ton of stuff in and out later, anyway). When you set aside time to write, you need to write. Turn off the Internet if you have to.
The only exception is outlining; if you hit the end of your outline and come skidding to a halt, then yes, it’s okay to stop writing narrative to brainstorm and lay out the outline for the next part to write. (At least, I don’t usually write outlines for a whole novel or story, but usually in smaller, lesser intimidating segments that I can check off.)
Also, if what you end up writing isn’t what you initially planned or outlined, that’s okay. If you end up meandering off, then realize that it’s not going to work and you need to drag it back to the plan, that’s fine too. It all depends.
Oh, and when you’re writing, it’s important to remember: YOU ARE GOD. Nobody else matters. Your characters don’t tell you what to do. If a character isn’t working out, smite them. You’re God! You can do anything! You don’t answer to anyone! Your characters are troublesome little creatures that don’t know a tiny fraction what you do, the audience doesn’t know, only you know (mostly) all. Write with arrogant conviction in your own judgment, write with gleeful enjoyment of your total omnipotence, with magnificent indifference to all others. You can edit as a mere mortal.