My writer's tip isn't exactly my own, but a customized version of advice I came across from Raymond Chandler: "When in doubt, have a man come through the door holding a gun."
What's important about this is neither the man nor the gun, but something happening. When you find yourself in the middle of a scene - or a book, for that matter - and things seem stuck or stale, introduce an unpredictable element to the action. A bird flying into the window, a pounding from the floor above, a man emerging naked from the woods. It doesn't really matter what the interruption is, so long as your characters find it dramatic, upsetting or intriguing. Because once you have your characters reacting to something new, your scene will be alive again. What's more, the way your characters respond to the interruption will often be revealing of an aspect of their personality perhaps even you weren't aware of. Making something happen makes other things happen.
Andrew Pyper's The Demonologist, was an international best seller. His most recent novel is The Damned.