Sometimes it's easy to overlook the obvious. Many aspiring writers spend a lot of time contemplating the perfect writing location, hopping from café to café, from desk to couch to kitchen counter. We trade productivity secrets and "hacks" like the Pomodoro method and Mac Freedom.
Get a comfortable chair!
But how much thought have you given to your chair?
As they say, the secret to being a successful writer is "butt in seat." It's as simple as that. But too often, I find that people cheap out when it comes to the actual seat. It may be a repurposed kitchen chair with a hard wooden seat and no back support. Or a clapped out office chair that perpetually sits three inches too low.
Café chairs are just as bad. I have never found a single one that provided proper lumbar support. But I understand that many people feel they can only tune out their day-to-day distractions when they leave their home, so that may provide enough value to outweigh the discomfort.
Think about the chair you usually sit in to write. Ask yourself, would you sit in that chair to read a book or watch television? For how long? If you couldn't stand to watch TV for more than half an hour in that chair, then it should not be the chair you try to write in.
The less comfortable your writing chair, the more distracting it will be. Nothing disrupts your mental flow like physical discomfort. A bad chair can make your feet restless, your back ache, and your arms tired. None of these things are conducive to getting proper writing done.
Your chair should be comfortable enough for you to sit in for at least an hour without getting pinched or bent or achy in a strange spot. (And this is while seated upright at the keyboard, mind you. We're not talking recliners, here. You need to maintain proper typing posture.)
If this describes your writing chair, then do yourself - and your career as a writer - a favor and buy a new one. You can pick up a reasonably decent (if bare bones) office chair for around $50 new at an office supply store. Even cheaper at a thrift store or on Craigslist.
Look for something that a secretary would sit in. Avoid the allure of the mock leather "executive chair." I have had many of these, and none of them is comfortable for more than ten minutes at a time.