@Kevin Trinh my only direct mail tricks are 1. Run list through USPS verification system to remove duplicates.
2. Run through USPS verification system to get deliverability status. Save your postage and don't mail to anything non-deliverable.
3. Consider sending a text message before the letter. Say something like "hey john, I put something in the mail for you and may have forgoten to seal the envelope, wanted to double check you got it?"
4. Test the **** out of your response rates and make sure your making changes to your letter as needed.
5. Your list source is Everything! How old is your list, are you removing people who have sold, etc.
6. Gold in the returned mail, make sure you are getting those addresses and doing a second search on them. If you cannot mail to original address than neither can anyone else.
7. How often are you hitting the same contact, consider changing up the piece you are mailing and the timing between the pieces depending on owners motivation level.
8. I am not a mailing house so saying this in all sincerity, consider how much its costing you in Time to do those mailings versus using a 3rd party. Time is all we have, what is your time worth? Can you use your time growing your business if you are not touching envelopes? No judgement here, just food for thought.
9. Personally I would stay away from "double dipping" as you put it. Stay focused on one niche, stay personal. Create the worlds best "home insurance letters".
10. Going to repeat KPIs again because it's by far the most important thing on this list. Make sure you are measuring the hell out of your response rate but most importantly how much is a deal worth and how much did it cost you (include your time) to get that deal.