multiple output

I apologize for asking something so basic --
I just can't find the answer in my books.

I've got a large, wild (meaning:  no schema or DTD) XML file
and an XSL stylesheet I wrote to process it into a standard XML format.
But the standard wants it to be broken up into individual files,
one for each recurance of a particular high-level element.
How can I best make this happen?

tx,
John