I've been hoping someone else would answer this, because I could use some new techniques.
Depends a lot on what kind of gap you're trying to fill -- is it that you know what kind of end you want but not how to get there, or is it a "yes, but what happens NOW?" Some tricks I've used include:
- Make a list of 20 things that could happen. Don't worry about whether they make any sense or how to make them work. The first few will probably be predictable, a few others will probably be too nuts to use, but somewhere in there may be the pointer you need. Or several things will combine into a good idea.
- Pick out some music that seems appropriate to your story, put it on the car stereo, and go for a drive.
- Go for a walk, and explain the story-so-far to invisible people who aren't there. (This works better if you don't live in a high-population area.) Imagine their reactions, have arguments with them about what the story means, etc.
- Take a shower. All the best ideas come in the shower.
- Go back and look closely at the beginning. What did you set up there that should be echoed at the end?
- Try to work really hard on something else very important that requires your full concentration. (That's when
my brain always decides to get with the creativity, anyway.)
- Try a random idea generator like Oblique Strategies, or open fortune cookies in the persona of your main character and interpret them accordingly.
Mostly I just bash my head against the wall. Eventually one of us falls down, and hopefully either the blood spatters or the rubble form shapes that give me a clue....