The trial-and-error has revolved mainly around the use of key search terms, something that is not at all intuitive for me. Once I became better at choosing (guessing) things like:

which words to use,
in what combination,
when to include/omit quotation marks,

my search results improved tremendously. I have also learned to be more creative with the terms I choose. Partly because FT declares so many words as too common to search, and partly because FTers frequently choose strange names for their thread titles.

If you are doing everything else you mentioned (limiting to one forum, etc) then hopefully trying a larger variety of key terms will make your searches more productive. Best of luck.