"Ignore it." -- Jack Smith, L.A. Times journalist, at a writing conference I attended many years ago.
If I'm stuck, I hang in there at the keyboard for a while. Sometimes walking away from it for a bit and doing some non-brainy activity, like plucking my eyebrows or folding laundry, gets the juices flowing again. If that doesn't work, I try, try again the next day. The important thing is not to give up.