Piglet noticed that even though he had a Very Small Heart, it could hold a rather large amount of Gratitude. A.A. Milne
I have a friend who fills at least two pages of A4 paper each time she's been to stay. She is hugely busy - two boys and a full-time job - and I have no idea how she finds the time. She goes into great detail, never repeats herself and is always very funny. (Frankly, I'm amazed she can remember her weekends with us, as she and her husband are always very thirsty and it takes all our stamina to keep up).
My point is though, her letters are thoughtful and beautifully written. And even though we have to replenish our cellar after their visit, they are welcome any time.
As children, we would always be made to write for absolutely everything - no matter how big or small. Visits to old ladies in the village for limp egg sandwiches and watered down orange squash; playdates with friends whose ex-military fathers scared us witless, or being given regifted lavender drawer liners for Christmas. It was always a bit of a chore. But now, funnily enough, I get my girls to do the same (although, they've managed to miss out on the pleasures of afternoons tip-toeing round a house, hiding from a red faced colonel who doesn't like loud noises or small children). And, miraculously, through osmosis, they both write pretty good letters.
Making an effort is always good. The "Dear Auntie, thank you for my present, love from So and so" whilst better than nothing, is never received with huge enthusiasm. With a bit of thought and imagination, it's entirely possible to flesh it out slightly more. Mentioning the present itself and what you've done with it, whilst seemingly obvious, always works. For the less experienced or linguistically challenged, it can be hard to pad it out. But simple ways such as writing about how lovely it was to catch up, or the excitement of a future visit will fill a line or two and goes halfway there to making it sound that it really is from the bottom of your heart. The greater the gratitude and effort taken in expressing it, is usually rewarded at the next present buying round. And likewise, when no letter is written, that person is in danger of slipping from the list entirely. My mother had a three strike rule. Of course, if the present is truly awful, then lying is the only response.
And it goes without saying, always hand write your note on beautiful stationery.