If someone has helped you out, it’s polite to acknowledge it. But it's also very good for business.  Here's why:

1. It's rare for businesses to send out thank you cards, so if you do, it really stands out.
2. If it's a quality card there's a good chance that it'll stick around for a while and won't be immediately recycled.
3. It gives your client a talking point when friends or family visit.  It reminds them why they use you.
4. It could quickly and cheaply increase the numbers of referrals you receive.
   
   

Here are some consumer and business examples. Search online for lots more ideas.

Some guidance on writing a great thank you note

1. Be sure to mention the client's name
2. Express your gratitude, clearly stating the reason for sending the note
3. Actively encourage them to continue giving you referrals (don't ask, don't get)
4. If possible try to add an incentive, discount or other special offer on the reverse of the card. That way your referring client has a great reason to hand it on to a friend.
5. Subtly include your proposition in a sign off statement so that they know why they should refer you, for example "Thank you for recommending us.  Our objective is to reduce the insurance costs for every single one of our clients and we're delighted to have done that so successfully for you.' 
   
   
   

“I’ve tried every referral system in the world but none of them seemed to work well for me.  As soon as I stopped asking for them and started thanking people for the referrals that we were getting, all of a sudden we started getting a ton more referrals.  It just builds on itself”       Craig Randell