A decent chair is a good starting point.
Set up your workspace, so that you don't need
to lean back and forth all the time.
Also make sure to switch positions during long sessions.
Play standing for 20 min every once in a while.
Search for a quick gymnastics programm that supports the
back and use tournament breaks for a quick workout!
Find a sport that will help you to develop a good posture.
DO NOT UNDERESTIMATE THE HARM YOU CAUSE
BY SITTING IN A BAD POSTURE!!!
Don't know how old you are. But i paid with serious back pain
for not giving a damn about posture when working at the computer
for hours each day. It takes some time before your body will "complain".
But then you suffer! A LOT!