First of all, find a room that's either pretty big, or pretty dead.
Upright pianos resonate from the sound board on the back. I've heard of people opening them from the top and trying to stick condenser mics by the strings, but that's ridiculous. Get two condenser mics on the sound board, about 1-2' away from the board. Also, the strings in uprights are almost always criscrossed, so there isn't much point in stereo panning with bass in one side and trable in the other. There won't be as much attack as there is with a grand, but you can still get a really dark, rich sound.