Don't let the dreaded Writers' Butt happen to you! The fitness of the largest muscle in your body is directly correlated to the fitness of your brain. Both body parts require exercise and blood flow to function optimally. Ours is a sedentary profession, so it is impossible for us writers to get as much physical activity as, say, a park ranger or an aerobics instructor. But ironically, the most effective way to keep mentally sharp and creatively juicy is to maintain cardiovascular fitness and overall good health. What you put into your belly feeds your brain, too.
We authors have a special need to make time to care for our bodies to keep our minds sharp and to keep pesky health troubles from interrupting our creative processes. My favorite exercise regimen is taking belly dance classes. I'm not a gym person. I like my exercise to be either so fun that it feels like playing or so practical that it counts as multitasking, like taking my child on a brisk walk to the park. Getting from Point A to Point B is a fabulous opportunity to stay fit. I'm always looking out for super-decadent, easy ways to eat healthy food and avoid toxins and environmental hazards.
The size and shape of your belly and butt don't matter so much as their health. You should have a solid hunk of muscle to sit on while you write and a tummy-tank full of good fuel. Your fingers can't type without your brain working, and your brain can't work without a functioning body. Don't end up like poor Steig Larsson and die before you get to experience your own success!