A guy who performs magic has been giving me some treatment. One suggestion he made was that I should set up a reminder on my computer to sit straight, as part of my problem is due to my posture.
This is how I did it on MacOSX.
1. Download and install Growl.app, with it's growlnotify command-line tool.
2. Find a small image you'd like the reminder to show (I use the xray of Homer's brain).
3. Install the plist below in ~/Library/LaunchAgents/org.fiveone.comfortable.plist
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<string>Are you sitting Comfortably?</string>
Running this from the Terminal, launches the reminder immediately.
$ launchctl load ~/Library/LaunchAgents/org.fiveone.comfortable.plist
This uses MacOSX's launchd.