The best method is usually to take a break. Sleep on it, or if it's too early to sleep, go for a walk. Fresh air always wakes my brain up and makes me see things in a new light. Having a bath also works for me, as well as reading other books in your genre. A last-ditch option is simply to start writing, even if the result is an unusable load of gibberish. Merely starting to write kick-starts the process.