This was usually a free report or booklet of useful information related in some way to financial planning. The postcard kept my name in front of them. Plus, every time they responded to request the new information, I had another opportunity to talk with them about their financial planning. Best of all, the discussion was always conducted under favorable circumstances because they requested the communication. Use postcards with just a brief message.
Postcards are easy to print and they're very inexpensive.
Send approximately the same number of postcards every week instead of sending a large number at one time every few months. You don't want to generate 250 inquiries this week and none for the next 3 months. The other example of successful follow up involves e-mail. Since retiring, I've written a manual and several reports that I sell on the Internet. E-mail inquiries go to one of my auto responders or come directly to me.