Rostering in general (from what I've heard, both from an ATC on the interview panel and an ATC friend-of-a-friend (so take with approximately 3 but no more than 5 grains of salt)) is largely organised by whatever group you're with - so for MEL enroute, basically the team that works that sector, within limits set by supervisors/FMS/etc and the need to cover the 24 hour period, but also with reference to when the busy periods are for that sector(s).
An example I was given is the 2/2/4(or is it 5? I think it was 4.) 2 mornings, 2 afternoons, 2 overnights, 4 days off which would look something like this:
Monday 06-14
Tuesday 06-14
Wednesday 14-22
Thursday 14-22
Friday 22-06
Saturday 22-06
Sunday not counted
Monday Off
Tuesday Off
Wednesday Off
Thursday Off
Friday 06-14 and repeat...
Of course, that's an idealised model roster. In addition to the (somewhat compulsory) overtime, people swap shifts to varying degrees depending upon the team and everything mentioned above.
That's my understanding at the moment. It could very well prove to be inaccurate!