I have always written thank-you notes, but have increased my pace due to John Kralik's inspirational book, 365 Thank Yous: The Year a Simple Act of Daily Gratitude Changed My Life.
What I've found though is that when it is a proper notecard versus an e-mail thank you, I need to set aside time for writing. I can't just scrawl them here and there.
One other lesson: The best is the enemy of the good. I've delayed some notes that will now have less effect than if I'd jotted an earlier but less eloquent message.