I was having difficulty with a story I am currently working on and it just wasn't coming easy for me. Instead of becoming frustrated I started working on another project; a non-fiction, that's been at the back of my brain for a few years. I kept finding information and tidbits I could use in this book so I set to work on it.
You might try this next time you find yourself in a rut. Set your current piece aside for a little while and take out one of the others you have started and work on it. Many writers have more than one story or project they are working on at any given time. You could even start a new one all together. Even if you just take notes or do research you will feel productive and will have put your time to good use.
Later, I'll pick the story back up with a new perspective :0)