In the subreport, create a *group* header (using the Sorting and Grouping
dialog), using a field that will not change part way through the report.
Set the Repeat Section property of this section to Yes. It will now appear
at the top of each page.
In a subreport, you can only do this with a group header, not a page header
(which does not work in subreports) nor the report header (which appears on
the first page only.)