If I do say so myself, I'm very good at sending thank you notes for monetary gifts, thanks to a little trick I picked up from an etiquette-conscious aunt:
I won't deposit the check or put the gift card in my wallet until I have written a thank-you note to the giver. It's a solid reminder, especially for cash.
Now, thank you notes for *things* often get forgotten. I'm really bad at that. I don't mean to be, and I feel bad about it. But sometimes you get to the point where it's been so long that the act of sending a note seems like it would only draw more attention to your atrocious lack of courtesy, and that the gift in question is likely forgotten. I can only hope that family is understanding about that.
(No, this is not a passive-agressive post directed at someone else. I'm just working through the piles of stuff on my desk, which includes a number of gift cards)