Your pencils are sharpened, and have have your outline. Or maybe your word processor is open and the cursor is blinking at you, ready for you to start writing. You stare at the screen. It stares back. It doesn't take long before your mind begins to wander. Wouldn't this be a great time to catch up on your e-mail? Heck, you might as well throw in a load of laundry before you get started. Soon you find yourself zipping around the house doing chores and starting home improvement projects you've been putting off for years.
Welcome to the trap.
I can't tell you how many things I've done other than write during my designated writing times. When things get hard, it's a natural tendency to want to do "something else," one you'll have to fight. The best thing I've found is, pick an hour a day for writing, and only write during that time. Even if you don't have anything to write, force yourself to sit in front of your computer the entire time. As you sit there staring at the screen, the words will come eventually. Ray Bradbury once said that stories came to him constantly and it was his job to be sitting in front of his typewriter when they did.
The less multitasking you can do during your writing time, the better. I have heard some writers will turn off their wireless routers during their designated writing time to keep them focused. If at all possible, maximize the window of your word processor, close your email, turn your phone on vibrate (or, heaven forbid, off) close the door, and WRITE.
Personally, I find the actual act of writing both physically tiring and mentally draining. My mind runs faster than my fingers. Sometimes entire stories will come to me in the fivue minutes I'm in the shower that will take me days to write. It's as frustrating as listening to someone who talks too way too slowly -- you know what they're going to say, but you still have to wait for the words to come out.
Once you start writing, keep in mind that it's never too late to brainstorm or adjust your outline. If a good idea comes to you while writing, jot it down and keep going. In my documents, I use a symbol to denote something I need to come back to. Typically I use three pound signs in a row (###). For example, if I need to research something further, I'll simply leave myself a brief note, preceded by three pound symbols ("###look up how to steal a boat"). Later, I can easily search for those pound symbols and find them again. Technically it doesn't matter what symbols you use as long as you use something that normally doesn't appear in your writing. I used to use "12345" but I find the pound signs stand out more and are easier to spot. Google Docs also allows you to leave comments in the margins of your work, and if you turn on the feature, you can track changes in Microsoft Word. I hate using track changes while writing a first draft, but I find Google Docs' note system very unobtrusive and useful.
Again, the key to writing is obtaining and maintaining momentum. Think of yourself as a train barreling down the track. The quickest way to reach your goal is to keep moving forward without making unnecessary stops or detours. As you begin to write, make note of what drags you away from writing and prevent those things from happening in the future. The Internet is my personal demon. I need access to it for doing research and accessing certain sites (dictionary.com's thesaurus, for one), but it never fails that an important e-mail will pop-up and derail me from my daily writing goal. When I write, I have to turn my e-mail off. Maybe I should consider turning off my wireless router, too.
Another trap that snares people is that they want to edit as they write. I have read many times that this is a bad practice. One theory is that writing and editing use two different portions of the brain, so when your mind switches to editing, it switches away from writing. Most word processors spellcheck on the fly so quickly that I don't think fixing those errors detracts from my overall forward progress. When it comes to bigger changes, I use those pound signs I mentioned earlier. If something's not fitting together or working out, I'll drop myself a note ("### - fix this part") and move on. Today's stumbling blocks are tomorrow's starting points. If you can't come up with the exact words or your writing is rough, who cares? Get it down and move on. There's plenty of time for editing later.
Do you write in your sleep? I do -- mentally, that is. I write all the time. Words and ideas come to me at the darndest times. If you have a bump in your writing that you can't get around, move on. The solution will come to you eventually. Trust me, even when you put down your pencil or walk away from your computer, your brain will continue to work on your book's problems. Like I mentioned before, that's where those recording devices -- be it a pad of paper or a voice recorder of some sort -- come in handy. Those "Eureka!" moments will come out of nowhere, and it's important to capture them when they do. When I get really stuck I like to go for a walk around my neighborhood. Sure enough, at the stop when I'm furthest from my computer, a solution will come to me.
Back when people wrote books on typewriters, most authors worked in a linear fashion (that is, they wrote the book in order from start to finish). In the digital age, this is not necessary. Both Commodork and Invading Spaces contain lots of personal stories and anecdotes. They are presented in a logical order (Commodork is presented chronologically; Invading Spaces is sorted topically), but neither one was physically written in the order in which the information finally appeared.
In the same folder on my computer where I kept my book, I also had a folder named STORIES. Inside that folder I had dozens of text files containing stories I had written for the book. Most of these stories were written based off of topics I had included on my original notecards. If I didn't feel like working on the book itself, I would instead write down one of the stories in its own document. Eventually I cut and paste the stories out of those separate text files and pasted them into the main one.
I'm not so sure this method would work well for fiction, as most authors tend to come up with their stories chronologically. That's not to say you won't have great ideas about things that may happen earlier or later in your novel. When those ideas pop into your head, jot them down and develop them when the time is right.
I wish I had more to offer on the subject of writing. All I can say is write every day, edit as little as possible, and push forward.