Always, my H and I have kids, so we have to interact more often than a lot of people. He noticed that I had put up Christmas lights last weekend, and I told him it was a lot harder than I had thought, and I hadn't realized how much effort he had put into doing that in years past. I apologized for not having helped more and just said I appreciated that he had done that work to make me happy. Then I changed the subject.
If you are contacting her periodicially to say thank you for something, you could try to very briefly tie whatever she did back to something in the past and briefly appreciate that. No superlatives, just a really simple message of "thanks, I did notice this before and it did mean something to me at the time."
