The theory is that blank fields on a line with no other content should close
up to clear the empty space. In practice this seems a little hit and miss
and therefore conditional fields can be used to impose your will. The
conditional field is used to place the line break and the field only when
the field has content.

{Mergefield Fieldname1}{IF {Mergefield Fieldname2} <> "" "
{Mergefield Fieldname2}"}

The line break is created by pressing enter after the quote mark.
If you wish to repeat for the next field, add it on the same line as the
last entry.
Field boundaries are inserted with CTRL+F9 and are not typed from the
keyboard.