In last week's New Yorker Patricia Marx (no relation, I think...) articulates the problem of remembering to remember as a lack of prospective memory. One technique Marx stumbles upon in her research is to "link the item that's threating to vaporize in your mind with another that's more rooted in your consciousness."
That is to say, put your house keys in the refrigerator next to your corned-beef sandwich (and then remember which to eat).
For this very same reason, I've kept a box of thank you cards next to my bed for the past two and a half months. Earlier this year I had discovered that extending my appreciation to those in my life was a habit I wanted to build into my life.
Of course, saying "thank you" is one of the first things we're taught as young 'uns, along with the rule of never putting your elbows on the table. Maybe its the fleeting nature of verbal gratitude or the immediate convenience of email, but showing one's appreciation through the archaic medium of pen and paper seems to land a bit more intentional for the the recipient. As for the writer's benefit, well, it can't hurt to take two minutes at the end of a day to consider those around you.
And if you do end up trying out this experiment, I highly recommend Blue Barnhouse's Tom Hanks cards: