Just write about how you feel, how your day was, what you want to do, what you're reading, describe your surroundings—whatever comes to mind. You don't have to read it, but it does help ease that blocked-up feeling you get. Plus, sometimes some pretty great stuff can come out of spontaneous writing like that.
I find it works for me. The feeling I end up with when that happens is like someone's put their foot on my hose while it's on, and the relief of writing a rambling few pages about nothing much is like the pressure's relieved. Don't expect wonders from yourself. Just write.
A lot of the time things like this happen because you have unrealistic expectations of yourself, even if you don't realise it. Nobody expects a first draft to be perfect, and even the best of the best (you know, those authors you think of with such awe it sends shivers down your spine) will often write total crap, probably more often than not.
Let go any ideas you have of grandeur—at least for now, and push that damn foot off the hose, and start writing.
In fact, when you've finished writing, put it aside. Don't reread for another few days, or even weeks. A lot of the time, when you reread right after, you can get frustrated with yourself. "This is crap," you say, and throw it in the bin. But maybe, after some time, allowing yourself to cool off and detach yourself from it, maybe it won't look so terrible.
I know that this has been a long, rambling reply, but I hope I've been of help!