take a cold shower, turn on the air conditioner, go exercise and come back and you'll feel more energetic, don't practice and rely on your looks (post alot under the good looking pianists thread), listen to music to gain your interest again, think of your piano as free therapy, invite other pianists to your house that will ask you 'what have you been playing?' (it's too embarrassing to say 'the same thing i've been playing.'), get some new music to use as a reward after you practice (this is the most motivating for me, since it is used as dessert, and sometimes i eat dessert first).