I take frequent breaks and do the Yes, No & Maybe so exercises! Nod your head up & down as if you're saying Yes, then move your head from side to side as if you're saying No then raise your your shoulders up & down as if you're saying Maybe! It really works or so the expert says that was on the Sewing with Nancy(Zieman) show.
You're not suppose to set longer than 45 min. at a time without moving around.