I've been writing some articles that are distributed on the internet, and so far the most popular one has been about how to get into the writing flow. I thought I'd share it with you here, too:
First, create an atmosphere in which you have as few distractions as possible. Later in the process, you may be so involved that you don't even notice things like a phone ringing but it helps if you can start off in an environment that makes it easy to concentrate. This also includes setting aside a period of time when you won't feel you really ought to be doing something else.
Choose a task that is at or just above your level of ability. If it's too hard or too easy, you won't enter flow. So if you want to write a novel, break it down into chunks you can handle. One task might be writing a rough outline. Another might be doing a character biography for your protagonist.
Finally, make sure that the task includes immediate feedback, so that you know as you go along whether or not you are doing well. For example, start by setting yourself a goal of writing a certain number of words per half hour. Eventually you may not need this kind of feedback because the task will so absorb you that you stop thinking about how you're doing it, or how well.
Ready to give this a try? Schedule some time during which you want to tackle a project and create the conditions described above. Go into the process with the idea that if flow occurs, that will be great, and if it doesn't, you'll still get a lot done (that mentality makes it less likely that you'll distract yourself by asking 'am I in flow yet?’).