If you want to really show your clients you appreciate them,  then one of the best ways is sending out a personal card with their image on the front and a personal message. I was hoping I could achieve this by using senoutcards.com but when i was checking out sendoutcards.com i was shock at just how confusing and expensive of a process they had. With their smallest package starting at $99 and the one you really want starting at $298, overpriced is an understatement. If all you want to do is send a handful of personal thank you cards in the mail every once and while to your clients, then take a look at http://www.notesbymail.com/. With the prepaid price being $1.99 a card, I think you'll agree they are more reasonable. Its simple to sign up and order. Some may think that sendoutcards offer more but who really needs all that extra stuff they offer? I don't, and I think most photographers will find that they don't either. Still in all fairness, if you order a lot of cards then senoutcards.com may come out less expensive, so you should really look at both sites and see which is more suited for you , but for me I'm going with notesbymail.com and not looking back.

PS. I've had correspondence with Ken Edwards (president and CEO) of notesbymail and he is making a custom back for me with my business logo and coupon code for their site rather than the default backing of the notesbymail logo. You can always upload you logo to other places on the card but being able to add this to the back makes things have a more personal thank you. He said that if more people contact him with interest in doing this he can automate the process. I say go for it, they have a great service and I think the more professionals that express interest in them,  the more they will be able to offer professionals.