I believe it's extremely important to give thanks when you have something to be thankful for. Whether it be the local newspaper running your press release even though they didn't have to, if you had a meeting with a client for the first time, when someone makes a small or large ad purchase, when you get a gift, for sending flowers.
Thank you notes have become a habit. I always send a follow up - or a quick hello in handwritten form so that those I work with and care about know I'm thinking about them.
That way you can never be forgotten and it shows your gratitude.
And I always sign with: Warm Wishes.