This letter is written for a dentist, but you can easily tweak it for any profession.
I used this numerous times, and everytime I sent it out I would get at least 3 phone calls for every 30 letters I mailed.
Here's the process I used:
Go to your local phonebook (yes they still exist) and write down the address of each of the dentists in your area that have decent sized ads.
Put their name, and address on a regular envelope (hand written). This will get it past the secretary since they think its a personal letter.
Put the direct mail letter inside with a $1 attached to the top (Halbert's famous tactic). Make sure you customize each letter with the dentist's name on it.
For about $45 I could get 3 serious phone calls with 3 different Dentists who were interested in working with me on local lead gen. That's a pretty good investment.
Here's the local lead gen letter that I wrote and used for all of this. Enjoy!