A simple and heartfelt message that reflects your gratitude and appreciation for the parents' love and support throughout the year is always a good way to start. Some ideas include: "Thank you for always being there for me, no matter what. I love you and I'm so lucky to have you as my parents. Wishing you a Merry Christmas and a Happy New Year." Or, "I know I'm not always the easiest to deal with, but you've always been there for me and I can't thank you enough. Love you both."