I never did anything in my frees, got in a bit of strife for it too I might add. I always did work in them if I had an assessment in the upcoming periods and an assignment with a closing-in due date, but I wasnt ever as consistant as I wanted to be.
During term 3 I worked out how to do abit more. I have 6 frees a fortnight, and devised that as a miniumum, I would alienate myself from all distractions and get some serious 50minutes of work done for 3 of these frees. This way I knew that if I wokred for half the frees, I got to chill and basicaly piss-fart about for the other half. I got into a routine and its virtually become second nature. Then I 'upped the dosage' to 4 periods, then 5 [etc]...
Now, I find it incredibly awkward if I dont do work in them, and an element of guilt kinda comes into it too lol. I dunno how your school works mate or if this can apply to you at all, but give it a shot, you never know. Besides, doing work in frees and extraordinarily beneficial.