Sometimes Luc, it's the actual ACT of contacting existing clients that triggers the action. I often get a "I've been meaning to get in touch for ages and then your letter came through the door" call.
Mike Halliday used to have some great postcards but if you cannot do that in the time frame (Xmas will soon be here)a nice friendly letter should do the trick.
The trick here is just to DO something, rather than worry for weeks about a letter or postcard being the best (I know because I have procrastinated many times in the past).