I remember watching one of Rosberg's videos about how he prepared for the season. It might help if I could dig out the video from somewhere (I think it's lurking around on Youtube somewhere, IIRC), but off the top of my head I seem to remember:
He has the personal trainer (as is normal today), who plans every workout with massive attention to detail.
He plays mental games during his workouts - for example, he must play pick-up-pairs whilst doing pushups at the same time. He also does things like catching a ball his trainer throws at him whilst doing other exercises, unicycling and juggling at the same time... all exercises that help concentration, multi-tasking, coordination and reactions.
He has a strict diet that he has to follow. He can eat anywhere he wants, just so long as he orders the correct food.
He likes to 'treat' himself to a burger at weekends.