Man, I can totally relate. Sometimes I have a real problem dragging myself to the piano after work and concentrating properly.
For me, the major problem is the feeling that practicing in such a state is daunting, so it's more of a psychological problem than a real problem. When I feel like this, I pick a piece to work on and then only work on one aspect of it, usually very slowly. Like, with a Bach fugue I've got to do, practicing hands together with a metronome and concentrating on getting each note perfectly in time. In some ways, playing that slow is easy, but in other ways, it's very hard to get the notes happening at precisely the right time. If I focus on just that one thing, I find that my mind doesn't wander away at all, and the next day, I can always play that fugue very fast much more accurately.
So, yeah, when tired, I just focus on only one thing at a time. I hope that kind of helps you out.
Oh yeah, and a cup of tea always does wonders for me!