I don't believe in writer's block. Not anymore.
Instead, I switch my focus to a different manuscript, which eases up the pressure and stress of working on the challenging one. Works every time.
I'm currently stuck on a self-inflicted writing puzzle (see previous blog post). My solution is to work on my sci-fi novel, which I haven't even looked at for a couple months.
It's refreshing, and I'm in no hurry to get it finished.
I don't know if this method works for anyone else, but it really does for me.