DO respond within three months of your wedding date or receiving the gift, whichever comes first.
DON'T email. Yes, it is easier. Yes, it is cheaper. No, it doesn't matter. Nothing replaces the thoughtfulness and grace of a handwritten thank you card. Look at it this way: In no way will you spend more time thanking them than they spent shopping for the present and mailing it.
DO keep track of what you receive. You might think that you will, of course, remember that Aunt Caroline gave you the gigantic crystal penguin. However, with all the stress and excitement surrounding a wedding, chances are that you may not. Keep a gift book with a detailed description of the present, who sent it, and when it arrived. You'll thank yourself for it later.
DON'T make excuses. If the three month mark passes, and half of your thank you cards are still not sent, don't use it as an excuse to stop altogether, with an embarrassed muttering of, "It's just too late." At the beginning of the note, briefly acknowledge the belated card, and then quickly move on to how much you loved their present.
DO mention the gift specifically. Simply writing 200 cards with the same message of "Thank you for your gift" will not do. Make each note as individualized as possible, and if you can, mention how you have already used or plan to use their present , as well as any anecdotes or personal stories (ex: Aunt Caroline, How did you know that Tom loves penguins?!). The only exception to this is a gift certificate or money. You don't need to mention the specific amount.