1. Buy your cards anytime throughout the year, whenever the moment strikes you.
2. Get your mailing list organized before you sit down to write your cards. Doing this can help you remember everyone and can also give you a sense of completion when you've sent a card to everyone on the list.
3. Timing is important. Send your cards in early December (late November if sending overseas). Sending cards to service members overseas? Plan on sending those right after Thanksgiving to make sure they arrive on time. Sending cards to business clients? Any time between Thanksgiving Day and New Year's Day is appropriate. While it's nice that cards arrive before Christmas, better late than never.
4. Write as much or as little as you want. Don't feel that you have to draft a long missive. Sometimes the shortest notes are the sweetest. At the same time, if moved to do so, indulge yourself and share as much as you want.
5. Don't stress, Enjoy! Put on your favorite holiday music, pour yourself a glass of mulled wine, make yourself comfortable and pace and enjoy yourself.