every client has different requirment even in same niche....
don't use same templete for writting cover letter,
because every business and job has different requierment..
so don't be salesy , understand job summery first ...
analysis your client requirment, what you can do for them and what not
set realistic goals... dont show them over night success ..
because your client is also human , and they can easily grab your fake and fluff words...