Growing up my mother had a clear vision on social etiquette including how to answer the phone, table manners, proper grammar, and how to write a good Thank You note.  Every time I sit down to write a thank you note I remember what my mother taught me and I add a little tip of my own. 
I will outline the steps I use to write a thank you card when I've received a gift.  I adapt the same steps to other situations where I use thank you notes, for example when someone has done something for me.  Feel free to use an adapt these steps for your own Thank You Card writing.  Have fun with it! 
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Example: Ava gives me a book as a gift.  
First - I thank Ava for the item, in this case it is a book.  Be specific.  "Thank you for the copy of 'Beartown'." 
Then - I share one thing about the item - What I will do with it.  Where will I put it?  How will I use it? What I like about it?  "I have heard good things about the book and look forward to reading it.  I needed a book to enjoy over the coming holiday and this will be perfect."
Fun Tip - Add a comment directly related to your relationship with the person you are writing to.  Something you are looking forward to doing together, that you miss about them, or something important to them.  "I hope you are getting lots of time in your garden this summer, it has been a good season for it." 
Wrapping Up - Before I end the note I say thank you one more time.  "Thanks again for the book.  It was a very kind gesture."   
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Here is is all together:

Ava, 
Thank you for the copy of 'Beartown'.  I have heard good things about the book and look forward to reading it.  I needed a bokk to enjoy over the coming holiday and this will be perfect.  I hope you are getting lots of time in your garden this summer, it has been a good season for it.  Thanks again for the book.  It was a very kind gesture.  
                                                                    Liz