another way is to suppress all the fields in the subreport...so it doesn't display anything, then in the main report place the subreport in its own section and in the Section Expert check the box to suppress blank sections.
CR runs the subreport, 'sees' that nothing is displayed and suppresses the section.
This solution was originally given by Brian, and oddly enough works like a charm ;)
Before finding this solution, I did as comatt1 did. This solution is easier to maintain as you don't have to worry about what is in that tiny section or where the subreport is if you need to change something.
HTH