My solution is to try to answer the question, "What's the shortest or simplest route between A and B?" And then I'll just start writing it, no matter how stupid or inane if feels. Usually enough starts sparking in my head to give me more ideas.

Another thing I try to keep in mind is something that I learned from a writing teacher a while back: In general scenes work best when one scene directly causes the following scene (I'm not saying this eloquently--essentially, they are causally linked, one following as the direct, and possibly only outcome of the previous one). So I go back to whatever is the last scene I outlined out and think, so now what happens? What will the characters do? For the time being, I don't worry about the more complex ideas or concepts, or even dramatic tension. I just try to role play the characters, what they are likely to do at this point. Usually that sparks something, too.

Middles are often about raising the stakes. They are about a struggle, a back and forth between protagonist and antagonist--it's important that they try. For the escalation of intensity and stakes (even if only internal conflict), they often follow an action-reaction-action pattern. I think of the middle as throwing stones at the character, complicating things, making life as rough as I can.

You know the goal, so think of what your lead is doing to reach that goal. Then think of what the opposition is doing to stop your lead. Then think of how the lead reacts to that, and what he will do after that.

I also ask myself: can the lead back out of the conflict? If so, I need to find a way to tie her into the conflict better.

Are the emotional stakes high enough? If not, the ending may not be satisfying or the setup may be weak. (No matter how hard she struggles to finish the Sudoku puzzle, we're not going to care unless there is some reason to care.)

Each scene should accomplish something. Either it's an action or a reaction, or it's a setup or a deepening. Setup and deepening, I think, are minor elements that can be worked into the larger action and reaction scenes.

Anything worth writing about is worth developing in a dramatic scene with things happening that the characters care about. If the protagonist has a bad marriage, and you don't want to write about that, understand that the reader doesn't want to read about that, either. Then look what Orson Welles did with that in Citizen Kane.