Often it is the case that a complex report becomes too "busy" to read easily.
Let's say that we are grouping by State, but it's difficult to look at the page and know where one state ends and the other begins.
There are many ways to alleviate this, but one of my favorite and simplest is to Insert Line.
This will insert, as expected, a line into the report which you can place wherever you want.
Typically, you'll want it to be in a Footer. This way, the line is drawn after each group is printed, effectively seperating the groups in a visual manner.