You've got your pencils sharpened and you've got your outline. 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. Soon your mind begins to wander. Boy, 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 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. It's a natural tendency to want to do "something else," one that 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 anyway. As you sit there staring at the screen, the words will come eventually. Break yourself of the habit of multitasking during your writing time and you'll find yourself more productive than ever before.
To be honest, I find the actual physical act of writing physically tiring and mentally draining. The problem stems from the fact that my mind runs faster than my fingers. I can write a chapter mentally in less than a minute, but physically typing the words out can take me hours or days. It's like listening to someone who talks too slowly -- you know what they're going to say, but you still have to wait for the words to come out.
While 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. One trick I use often is, I use a symbol in my documents to denote something I need to come back to. Typically I use three pound signs in a row (###). For example, if there's a place you want to later insert a picture or need to do further research later, I'll leave myself a brief note, preceded by three pound symbols. 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.
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 additional stops or unnecessary detours. As you begin to write, make note of what drags you away from writing and prevent those things from happening. 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. When I write, I have to turn my e-mail off.
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. (Honest disclaimer: I do it.) 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 processing programs perform spell checking in the fly so quickly that I don't think it detracts from my overall forward progress. When it comes to bigger changes, I use those pound signs I referred to 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.
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 the problem. 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.
Back when people wrote books on typewriters, I think many 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 written in the same order in which the information was presented. When I began writing Commodork, I had chapters that looked like this:
"Then there was the time I was almost busted by the FBI. (###FBI Story) That was a close call and I barely escaped prosecution. Too bad my friend The Boss wasn't so lucky. (###Boss Bust) (###Scan in Boss Newspaper Article and include it here.)"
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 stories written for the book. Most of these stories were written based off of topics I had included in my original outline. 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 was able to simply cut and paste the stories out of their own separate documents and into the main one.
I'm not so sure this method would work well for fiction, which typically is still written in linear fashion. That's not to say you won't have great ideas about things that should happen earlier or later in your novel (and again, when those ideas pop into your head, jot them down!).
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.