Free your mind and the words will follow
The Wordpress Daily Post a while ago mentioned the use of free-writing. Now I've never actually called it that but I use it all the time either to overcome a bit of writer's block (fear of committing myself to a paragraph that doesn't sound spot on first time. Even though no-one needs to see it in that state and I can change it at any time) or to fill in 5 minutes when I'm at a loose end. The idea is to write for a fixed amount of time without lifting the pen (or pausing in your keystrokes), writing repetitive nonsense if necessary, but hopefully loosening up and producing some coherent sentences.
I first came across the idea a few years ago at a most enjoyable short story workshop hosted by Mandy Sutter (I think her story The Therapist is available on her site - worth checking out, rich and evocative but laid-back, kind of Alan Bennett-ish perhaps). She used it as an exercise to free us up and get the words flowing, but she also suggested using it when you're stuck somewhere in the middle of a story - take the last line you're happy with, then write for 5 minutes without pausing and see what you come up with.
I do random 5-minute efforts all the time; I choose a first line by opening a book at random (fiction or otherwise), or using the first line of someone else's story or article, or even some phrase I've heard on the radio. OneMonkey occasionally compiles short lists of opening lines for me to use (sometimes with mix and match genres, which is fun), and one of my recent exercises involved 5-minute bursts (which is what I usually call it, rather than free-writing) all starting 'It all began with...' plus some unlikely object (a cardigan, the table-lamp). It's amazing how much variety that produced.
More often than not the product of my 5 minutes of writing is nonsense, it doesn't flow, I've maybe missed a word out. But even then there may be one line or one phrase that strikes me as quite neat, and I put it to one side. It might even suggest itself as the title of something. On luckier occasions (or at times when I'm firing on more cylinders, at least) I'll get something that's a decent first draft for a piece of flash fiction, or a good opener for something longer. Enough in the way of useful output that it's definitely worth continuing (plus it's quite fun to be liberated enough to write freeform gibberish for a few minutes. I'm too uptight and nervy to do that most of the time because - gasp- what if someone saw it...). So if you've never tried it before, sit yourself down with an egg-timer or something, take up your pen or keyboard of choice, and get going.