More times than not, your customers throw away your business cards. They are figuring, "I just purchased my home and won't be needing a real estate agent for a while, so I'll just toss this card." Eight months down the road when their brother is looking to buy a rental property, your former customer will have trouble remembering your name, let alone your phone number.
Here is what I would do to add so much value to your business card that he wouldn't think of throwing it away. At the closing table, take out a business card. On the back of it, handwrite your personal home phone number. Tell him or her, "Here is my business card. On the back of it I have written down my home number to call in case there are any problems. Most of my customers keep the number in their wallets so that they know exactly where it is if they ever need it."Will they ever need to call your home number? Chances are they won't, but they won't throw that business card away because at some point they might need to use. And they realize that it is much simpler to keep that info in their wallet than to go looking for old documents. And because they won't throw it away, 8 months down the road when their brother wants to purchase, your customer will have your business card handy.