Recently, I've been combining a couple of my 2012 resolutions. In between writing more, I've been taking mini-breaks during my writing sessions to get active -- even if it's only for a two minute stretch.
When you get stuck for a plot idea or a line of dialogue, instead of staring at the screen trying to beat your muse into submission, try getting up out of your chair. Do a few jumping jacks, crunches, stretches or other physical activity. You could even do a little frustration dance.
Taking a break out of your writing day will burn a few calories, which is also one of my resolutions this year (and every year). It will get your blood flowing and minimize the risk of 'pins and needles' or even something more serious. Better still, that blood will flow to your brain, maybe even resulting in a new idea. Hey, I often think better when I'm doing something totally mindless.
So break out of your writing cycle and jump on a real cycle. Or just jump around for a few minutes. You will both feel and think better.