Please don't make the mistake I see in so many stiff and unkindly worded Christmas cards: "... we thank you for the pleasant cooperation over the past year and hope to welcome you back as a customer in the new year..." - Ugh! Preferably printed and then signed with an illegible pen. There is no better way to ruin this very personal and emotional marketing method.
Writing isn't actually that difficult. You're not supposed to write a letter, just put a few nice lines on the cardboard.
I have found it useful to start with a personal belize telegram screening greeting. Whether you remain more distant and write "Dear Ms....", take a more relaxed approach with "Hello Mr...." or underline an already very good customer relationship with "Dear Ms...." is not crucial to the success of the card. Just make sure you use the recipient's name. If you are on first-name terms with your customers anyway, everything is a little easier.
Then there is a line explaining why you are writing this card, garnish the whole thing with a suitable saying (you can find good sayings here or here ) and then you can add the farewell greeting with your signature. That's it! That's all you need to make a positive impression.
To be clear again: Do not use these cards to make an offer to your customer! You only want to remind them of you. If you include a voucher, an offer or even just an invitation to come and see you again, you will immediately cancel out the positive effect (“the lady from the flower shop is really nice, she wrote me a card”) (“Oh, just advertising!”).