I would start out in a very conservative manner now. Remove the lowest tier of branches to expose the trunk. This might mean pruning off just two or three branches. Of course, cut them all the way back to the branch collar...don't leave stubs NOR cut completely flush to the trunk. Next winter (late winter) limb the plant up a bit more. Do not do this kind of pruning in the summer or fall.

You'd get better information if we knew how large your tree is, or if we could actually see some images.

Hi Rhizo,
Thanks for your response. I'll try to figure out how to do pictures, but in the meantime I would describe them as being about two feet tall with three to five branches all shooting up from the base. I'm guessing I'll have to choose one to be the main trunk at some point (when?) and cut the others away down at the base, but none of them stands out as better, bigger, stronger or more erect than the others. And I'm not sure when it's best to trim other potential trunks away, leaving just one. Does that make any sense?