I participated in a panel at LOSCON called "What I Do When I Should Be Writing." The confessions included the usual: blogging, emailing, eating, shopping, channel-flipping, furniture moving. One surprise: a lot of us find that washing the dishes (really) helps when we get stuck on a writing project. Warm soapy water, a pile of clean dishes, and all of a sudden, the phrase we were looking for or the concept that moves the plot along just floats up to the surface and is there for the taking.
One of the most important comments was the simple affirmation that everyone, and that means everyone, has a hard time committing to seat time, to actually getting into the chair and staying there long enough to actually produce text. The best advice? The last thing you do every writing day is to make sure that it is super easy to get started the next day.
One way to do that is to set it up so that you are twitchy to get back at it. Stop in the middle of a sentence. Deliberately typo a few words in the paragraph. Sketch out the scene in a quick draft so that all you are doing first thing next morning is siting down and filling in details.
For me, the key is to leave specific instructions for myself on a sticky note ("START HERE!"). To make sure that the first task of the day is small and easy. And to make sure that all of the materials I need (books, papers, calculator, sharp pencil, whatever) are set out at the writing desk.
When you know what the next task is, sliding into the writing day is like sliding into your comfy slippers. When all your materials are assembled, you are less likely to shatter the flow by rumaging around for some piece that you need.
Another good point that came out of the panel is that fiddling around (aka procrastination) can sometimes be good for a project. Instead of banging your head against the keyboard, it might be more productive to go for a walk, work in the garden, run an errand. The key seems to be to keep it short and also to avoid words: walking is good, tv is not; pulling weeds or taking a shower or organizing a closet is good; reading a magazine is not. Set it up so that the word-making part of your brain is still simmering on the project while your physical attention is turned to something else for a bit.
Just be sure that you are really clear about the difference between incubating an idea and avoiding one.