1. Let kids pick out their own thank you notes or cards. Take them to the store and let them pick out whatever they want, even if you think they're dreadful. It helps them feel ownership about the project. You might also consider having personalized stationery made for them or, if you have the time, having them decorate blank cards.
2. Make it a family activity. You surely have your own cards to write so set an example and have the whole family do it together. Put on some music or even a movie and hunker down until it's done.
3. Older kids probably won't care, but young kids are fascinated with the mail so make sure to let them help seal the envelopes, put on the stamps and take them to the mailbox.
4. Reward yourselves with a fun activity. It's not a bribe, but a reward for a task completed. Go for a hike, go bowling, go out to dinner, go to a video arcade, whatever is fun for everyone.
(Image by Flickr member daBinsi licensed for use under Creative Commons)