Dread of sitting down at the computer
Serial procrastination: almost any odious task is preferable
Constant checking of the clock or word count: can I stop yet?
Guilt. That's a big one.
Anxiety and self-doubt: I don't know how I wrote that last book, but I'll never be able to pull it off again
Long periods of staring blankly into space
Finally writing a single sentence, reading it over, and then deleting it because it's horrible
Deciding that the entire concept of your manuscript actually sucks and you need to start again from scratch
1. Get outside. Do something, anything, that unfetters your brain and lets it wander. My best solutions and plot fixes have come on runs and bike rides.
2. Go back and re-read your manuscript straight through from the very beginning. You don't want to do this too often because then it makes it harder to have fresh eyes for editing later, but when you've lost your way, it can really help you get back into the voice and characters and pacing.
3. If you seem to be stuck on one particular scene, jump ahead and write a scene you're looking forward to, that's exciting. Write the end, if you want to. And sometimes the scenes I get bogged down in are ones that I should probably lose anyway. If I can't even summon enthusiasm for them, who should I expect a reader to?
4. Sometimes it isn't your imagination; it's a very real problem in the story that a nagging little voice in your head is aware of and that you need to listen to. So identify the problem. What just worked for me was writing a new, one-page summary. I realize there just wasn't enough suspense building toward the final third. The fix required some work, but it was completely worth it in the end.
5. Walk away from the computer and read! Explore a different genre, discover a great new author. Look at what works in the story and what doesn't. If you just read something you couldn't put down, figure out how they hooked you and see if you can translate that into your own story.
6. Finally, whether you are working on a self-imposed deadline or one set by an editor, take a breath and remind yourself that this is a first draft. You can—and will—go back later and polish, adding scenes, deleting others, and generally reworking the whole thing. It doesn't have to be perfect. It doesn't have to be the best thing you've ever written, and in fact you wouldn't want it to be, because that means it's all downhill from here, right?
Happy New Year, and happy reading and writing!