I find that inspiration comes to me normally in the morning when I wake up, usually after a heavy fic-reading session the night before. I'll just have a random scene pop into my head in the ten minutes before my alarm goes off...and then I have to get to work. How damned inconvenient. So usually I end up trying to summarise what the scene was about in a reminder in my phone for when I get home
Then of course, it's a case of getting it all down on paper. I've got a bit of a library of "scenes that haven't happened in my fic yet" starting to build up. I keep them in a separate document so that I don't get mixed up in the main fic document. I can just slot them in when they occur naturally that way.
Funnily enough, I've found that sometimes it is bad to over-plan. My first attempt at a fic got forgotten about because I planned it all out right down to the dialogue that would happen, but as a result it all just happened to quickly, there were no 'in-between' bits, no filler scenes etc. I just got bored of the story because I knew what was going to happen next, y'know?
Recently I've been giving myself a vague outline and just writing spontaneously to see where it takes me. And then when I get stuck for inspiration, I start to work in one of the 'not happened yet' scenes. It's kept me going so far, we'll see how long it lasts
But yeah, reading (anything, not just fic), or even watching series 4 again (for the the thousandth time - you always notice something new every time you watch it, no matter how well you think you know it) does wonders for my inspiration.
Just ten penneth.