That's a tough one. Often the only thing that works is time. If I'm having trouble, I'll stop writing and start reading, either other author's books, or I'll revisit partial manuscripts. I'll re-read the manuscript and see if something doesn't shake loose and work on that for a while. Sometimes I just trudge through and force myself to write. Then I go back, read what I've written, and usually my editing pen comes out. After that, things often click into place. It's different for every writer.