Go to the Layout Editor and add a horizontal line to the layout. Place this
line directly below the field that will be associated with the format trigger.
The line should also be within the repeating frame of the referenced column.
Apply line color to make the line white (so it won't show up in the printed
report) and set the properties to Page Break Before.

Then add a format trigger on the line object to check the value of the desired
column. For instance, if a master-detail report is created on the EMP table
and a page break is desired whenever the value of the COMM field is not null,
create a trigger as follows:
When the report is run a page break will occur whenever the value of COMM is
not null.