My muse is a very fickle mistress - sometimes she sings so sweetly it's impossible to get the ideas down fast enough. Other times she won't croak a word, which is damn annoying, because it means I have to do it myself. And so often I'm a procrastinator. The best advice I ever had, and it works really well for me is this: when you hit the wall, you've got to kick-start your mind, so it sees a way to climb over it. It won't do that if you stare at the wall (your work-in-progress) trying to force it, so start writing something else, something totally unimportant, just something you're writing for FUN. It'll never see the light of day and it could be porn, it could be horror, it could be Anne of Green Gables meets Frankenstein. You don't edit, you don't rewrite, you just let the words come out of you and you laugh at your own ridiculousness, and enjoy yourself.
I think it's akin to an engine that is barely ticking over, and then it catches, the revs rise a bit, but smoke pours out the exhaust (muffler to you) and you coax it along, and it gathers speed. Getting it going was what was needed, no matter how, and now the wind rushes in your face (I see it as a motorcycle, myself) and you start to really enjoy it. Your brain has been encouraged back to its optimum state, and now you switch vehicles and go back to your wip. Do this as many times as you need to...
One of my writersblock works has turned into another wip, which I still go to when I have brain lag...