I dont send cover letters just a simple one page resume. I dont even put an objective on my resume just my ratings, times, work history and education. They are all 135 charter outfits so I assume they know what I am after.
If its a company I know is hiring and I meet their quals I also include my certificates and my Simuflite cards.
I think the key thing besides flight times is experience and currency in the aircraft type they operate. That why I suggested only sending resumes to companies that operate the aircraft you are qualified in.
If you have a current 8410, are typed and have some type of Simuflite or FlightSafety currency it shouldnt be too hard to chase someone down that needs a pilot.
I have had a much better response sending unsolicited resumes than applying for jobs that advertised on AEPS, climbto350, etc. Probably because I wasnt competing with 10,000 other resumes
You can also try looking up owner registrations on the FAA website or aviation databases for a particular make and model. There are probably 1,000 King Airs out there. The downside to this is most turbine aircraft are leased and the registered owner will probably be some bank in Delaware