For me, a paragraph just dropped into my head one evening when, funnily enough, I was walking to the shop to buy some milk! I don't know where it came from, but I knew it would be the last paragraph of my first chapter. Then I was stuck for over a month. I knew my two main characters, had them fleshed out as people, but I didn't know what I was going to do with them. I just kept hitting a blank.
Wool is what saved me.
We've moved house an awful lot over the last few years, and one evening I was down in the cellar looking for a particular ball of wool that I was fairly sure had ended up down there, not in my knicker drawer, the toy boxes, a kitchen drawer, or any other number of wrong and inappropriate places. Reaching for a box, three bits of folded over paper fell out of an old diary. It was a letter from an ex, soon after we split up many years ago. I don't cry very much, but reading his letter on a Tuesday evening in my cellar made me burst into tears. And I started thinking about first loves, and how they become formative experiences. And then I thought a bit more about how hellish growing up can be. Then I thought some more about how our parents fuck us up.
Then I thought about it some more. And when I finally got bored of being too scared to write, I wrote myself a stern letter. Telling me that no one else in the world had to know I was writing, that I could just do it and then bury it in the garden. So I got on with it. And sometimes it flowed, and sometimes it didn't. But at the time I said to myself 'Sod it. Just keep writing, even if you know it's a badly constructed sentence. Keep the story moving, and edit it later.'
But the thing that worked was, just like Tunip, WALKING. Specifically, walking on my own to somewhere. I seemed to have my best ideas about plotlines just as I reached the corner before crossing the road to the DCs school, so made sure I had my diary and pen handy to scribble down ideas. Dialogue ideas/'moving the action forward without major plotting' ideas always seemed to happen when I went to the shop to buy milk/bread/wine in the evening (I deliberately buy smaller bottles of milk now).
Of course my book and the sequel I'm working on now might be terrible. But those walks, those eureka moments are so satisfying that even if I never get published, I'll keep on writing.