Sandyolmt,
I send "Thank You"'s whenever I want to make an extra impact. I send all of my first-time clients a handwritten thank you note and enclose a "frequent flyer" card that they get initialed each time they visit (6 times earns a free 7th) and that lists my referral rewards on the back.
Then anytime a client does something special (like a gift) I send a handwritten thank you. I have to qualify that by saying that I wouldn't send a written thank you for a tip - ever. Tips are for services rendered or for going above and beyond, etc. and don't require anything more than the initial, verbal thanks. Gifts are tokens of appreciation or affection that show the person went out of his way to choose, purchase and deliver his sentiments materially. And you should do the same by sending a card.