If I'm really stuck on the next moment in a story, I'll take it to bed, and as I'm falling asleep I'll play the scene in my head. Usually I'll find a direction that way. If it's just a matter of I don't have the concentration to write or figure out a transition into what I know I want to say...well, the only cure for that is to sit down and do it. Even if it's half a sentence. Sometimes a few words will get me over the hump.