i put a mirror on the rack so as to check my shoulders if they're down, and i write out "RELAX" on that mirror to remind me always.
LOL, the idea behind the balancing a book on your head is, that you dont need to look in the mirror. Because any unnecessary Body movement will be intantly revealed to the pianist, since the book will fall on the ground
not really. a pianist has to relax but not remain seated like a statue so as not to make the book fall. any body movement doesn't necessarily mean one isn't relaxed. i can dance on stage and still be relaxed. on the contrary, a pianist may stiffen up trying to balance that book. the book may be a good practice for focus.
does it help to do some stretches before plaing, as in sports stretches, just to relax. i dont do this because i dont know if its right or not, but i generally feel more relaxed after stretching.