Here are a couple tips that have worked for me.

  1. Keep the envelope as plain as possible. I usually only put my return address in the upper left. I hand write (or have my wife do it) the addresses.
  2. Also, any way you can personalize the contents. For instance I'll print off a screen shot of their website or Google my business listing and mark it up with a red pen, pointing out issues. You can also add a handwritten post it note with something like "I can help - no obligation. Your name and number.
  3. Once I mail them I'll wait 3 days or so and do a follow up email or phone call to see if they received the letter and have any questions.