Sometimes I just have to walk away for a few hours, or maybe a day or two. I have also learned that when I try to force a story things will often get jammed up so I have to just relax and let it develop at its own pace.
One practice that works when I have writer's block is to just grab a piece of paper, sit somewhere quiet, and just start writing down every word that comes to mind. It seems like a pointless practice, but I have found that it clears out the congestion. I have also found the words I needed mixed in the scribbles.