When sending a thank you note you want to try and be as personal as possible and all notes should be sent out no later than 3 months after the weeding.
Make sure that the note is hand written and if the person gave you a gift other than cash you should mention it on the card and make a cute comment about how it has come so handy. Even consider including a photo of you and your hubby using such item.
Many of our couples get stationary printed at the same time they get their invitation done, this way its easy to start them as soon as you are back from your honeymoon. Remember to keep the note short and sweet.