The way an envelope is addressed is just as important as the content inside of the letter. Although you may be writing to your very best friend, it's important to be proper when addressing the envelope.
Couple: Mr. and Mrs. Husband's Name
Family: Mr. and Mrs. Husband's Name and Family | The Last Name Family (slightly more casual)
Single: Miss*/Ms./Mr. First and Last Name
*Miss is typically used for girls under 18
Another important aspect to the note is your handwriting. There is no point in writing a note that no one can read, so be sure to keep it neat. Despite the pink pen shown in the picture above, thank-you notes should be written in blue or black ink. Traditionally, they are written in cursive, but I don't see a problem with printing...as long as it's not to your grandmother or great aunt!
So what are you supposed to say in a thank-you letter? Well, the content isn't to difficult to come up with! Here's an example:
Thank you so very much for the Keurig coffee maker and drawer. They are perfect
and will keep me caffeinated as well as organized in my dorm next fall.
It was so great seeing you two at the beach last weekend, I wish we were able to get
up there more often. Hopefully you'll be able to visit us soon!
Thank you again for the Keurig, it will be so useful as I venture into my college years.