i once had a program that would remind you at set intervals to take a break from the computer - you know, for health reasons.
of course - i uninstalled it and continued to spend hours sat staring at my monitor all day instead. yep, i'm probably going to get deep vain thrombosis as a result of using my computer - great.
how about a little program that you set to blank the screen every couple of hours or less - maybe it bring ups a message (visual and audio) telling you in a polite way that it's time to go and make a drink or whatever, thereby forcing you to take a break.
yeah, i know you'd ignore it and just carry on working - the crucial part is the screen would remain blank for 5 minutes (with more messages or whatever displayed) and you would have no way of overriding it - any attempt to do so would hibernate the computer.
(i know, i know, i'd uninstall it and never use it - let me just dream for a while that i would do the right thing and obey it.)