don't. obviously.
i don't feel there is a point in practising if you don't feel like it. just a waste of time.
of course, if you feel like that all the time, then there's a problem.
I have a FIXED practice schedule which is from sunset to midnight - "the whole night" - so whether I
feel like practicing or not, I establish to myself I had to do so. (In this manner, I'm using more my HEAD (mind) than my mood; this is more rational.) For this reason, I'm always struggling and rushing to do all non-piano stuff before sunset knowing I would be "jailed" in my piano when this time arrives.
"Dazzer," you have a point in what you said above since we have a tendency to sound mechanical and "feelingless" when practicing "not in the mood" (and in the first place, music is about feelings; however, as far as I'm concerned, it's really UNSTABLE to rely on our BODILY MOODS (sense, emotion, disposition) and such is quite risky 'cause more often than not, our body is always after PLEASURE - and that is something NATURAL. This is unless you're a MASOCHIST who derives pleasure from pain. (BTW, the pleasure and pain I'm talking about here is PHYSICAL; thus, I might torture myself in the piano just to feel bad physically afterwards but "deep inside," I'm FULFILLED.
In conclusion, practicing the piano for me is often an ironic activity in which "the soul is willing but the body is not."
_________________________________________________________________
Music is the language of the soul. - L. van Beethoven