Humans are perverse. We want what we don't have. When I'm writing, sometimes my mind wants to be anywhere else. When I'm not writing, I want to be back home, typing away.
Mentally, I think we need other things -work, hobbies, etc- in our life that is not writing to challenge other parts of our brain. I seem to be a lot more productive when I'm working than when I have a month off to 'just write'. I think being away from my keyboard gives my subconscious a chance to mull things over. Work forces me to use other, logical parts of my brain, letting my creative side rest, recharge and ready itself.
Make your Muse (or subconscious if you perfer) chase you. Have something to do that doesn't involve sitting down in front of a computer. Get out of your house, DON'T write for part of your day. You might be surprised at what comes rushing out when you get back in your Throne of Writing(tm)