A common phrase for a thank you card is, "Thank you for your thoughtfulness." Another example is, "All I can say is thanks!" If the note is for a gift, one can say, "I was touched when I opened your gift, you left me speechless! Thank you so much."
One important thing to express in a thank you card is acknowledging the card receiver's thoughts and feelings. An example would be, "Thank you for thinking of me and for being so generous." A compliment about the person is also a great way to acknowledge the person, such as, "You are such a great friend to me."
If the card is a thank you for a gift, giving an example of how the gift has been used is a way to let the card receiver see what the gift means to the card writer. For example, if the gift is a gift card, the writer can say, "I used the gift card to purchase my favorite perfume." Finally, the writer closes with a statement of gratitude that expresses how the he was personally touched by the card receiver. One example is, "Your generosity touched my heart so much that it made me cry. Thank you, again!"