After the holidays, I think it's always important to reciprocate a gift, big or small, with a nice hand written thank you note. It lets people know that the gift they spent time picking out for you was important, and that you're grateful for it. Also, who doesn't love getting a little something in the mail?
According the Emily Post, a thank you note for gifts given for Christmas or Hannukah should be sent before New Years Day. However, given how hectic the holidays can be, I think that any time during the first week of January is an appropriate time to respond to a gift with a Thank You note.
With that said, it is also important to be polite and gracious when opening presents in front of the gift-giver. Always say thank you, and never allude to the fact that you don't like what you've been given or that it may be a duplicate of something that you already have. If you do receive a gift and open it in front of the giver, then a written thank you is not obligatory (according to Ms. Post).
When it comes to writing the note itself, make sure to always reference the gift given. Even if it is something truly hideous, note how "unique" the piece is. Also, try to find a way to mention your gratitude for the time and effort spent picking out the gift. I think that everyone always thinks they have picked out a good gift, and hearing some appreciation for that is always nice.
Until next time,