No matter how ready you might feel when you put your pen to paper (or fingertips to keypad, as is increasingly the case), chances are at least part of your conscious mind is still a part of the real world, instead of the one you're about to create. Whatever you're writing about, start off getting rid of everything else by freewriting.
Have a scrap notebook just for this and, before every writing session you have, fill a page or so with anything on your mind. Get out anything that distracting you by writing about that first, even if the writing is bad and the content is boring. The bad and boring writing can be discarded before you get down to doing what you set out to. Maybe freewrite about what you wanted to write about in the first place; if nothing else, it'll feel a bit clearer and more defined when you really start.
Even if you don't notice it immediately, this little trick - which can take as little as five minutes, or as long as you feel to need to get in the zone - will loosen you up before you get stuck in.