Are any fields set to "slide up"? If so, then there's no easy way to avoid this. You can specify that a part not break across page boundaries, but you don't have an option to keep all body records with a preceding sub summary part (A most desirable feature folks can request if they want: http://www.filemaker.com/company/feature_request.html )
If you don't have any sliding taking place, it is at least theorectically possible to calculated where page breaks will take place and then it is sometimes possible to add one more field to your table that computes a page number and a small blank sub summary part "when sorted by" the page number field to force the page break just before the next sub summary part starts a new group of records listed in the body. This is not a trivial excersize, but it can be done.