As an example, let's say you want your subreport to print after a group footer & before the next group header.
Insert a 2nd group footer & create/insert your subreport there. Once you've got the subreport looking the way you want (subreport margins, for example, can be a BIG pain), right-click it & select "Format". Click the formula button next to the Suppress option, then set up whatever sort of criteria you need to determine if it prints/runs or not. Then right-click the footer out in the margin & select "Format"; check "Suppress blank section".
So, at this point, you've set up the conditions under which your subreport will (not) appear; if it's suppressed, the new group footer won't show, either.
All that's left is to right-click the footer again, select "Format section" & play with the New Page Before/After options; you'll have to see which one gives you the sort of page breaks you want.