mm I think it has a lot to do with your playing style. Try not to bend your wrist too much. and also just relax when you play~ the stiffness will make you tired real quick.
and stop playing the moment your wrist hurts, you dont wanna risk it =)
edit : as for warm ups, just do scale runs, spider movements and all that. It's kinda boring, so what i do is choose a few songs, at just the right difficulty, and play =) just relax, and have fun. if you tense up too much, well to put it short, its bad in every way. so, relax~!