If you have, e.g.

{ MERGEFIELD Title } { MERGEFIELD Lastname }

then you need something like

{ IF "{ MERGEFIELD Title }" = ""  "" "{ MERGEFIELD Title } " }{ MERGEFIELD
Lastname }

If the Title evaluates to a space character within Word you may need
something more like

{ IF "{ MERGEFIELD Title }" = ""  ""
"{ IF "{ MERGEFIELD Title }" = " " ""
"{ MERGEFIELD Title } " }"
"{ MERGEFIELD Title } " }{ MERGEFIELD Lastname }

If you are using ADDRESSBLOCK and are getting spaces where the title should
be, you will need to change so that you use individual fields, as above.

Use a conditional field to insert the title and its following space eg

{IF {Mergefield Title} <> "" "{Mergefield Title}<put a space here>"}{Put the
next field here without a leading space}

Make sure that the fieldname above matches that in your data.