:: The other tip I'd add is just to try to keep the subject framed as evenly as possible throughout your shots.
You're right that using a tripod or monopod isn't always helpful when you're not dealing with a perfectly flat floor.
The best thing is to pick something to always center the lens on and keep an eye on the space above, below, and to the sides of your subject.
This is especially true in cases where the physical setting simply won't allow you to walk in a perfect circle around your subject (due to streets, walls, shrubs, trees, people, etc. on one or more sides which force you to walk a non-symmetrical path.
I know that we aren't meant to change camera zoom levels between shots in Photosynth 2, but I would rather use camera zoom to try to keep my subject framed in the same way and complete the circuit, if possible, than to give up because I can't keep the same distance all the way around in such cases.