Contrary to urban legend, you do not have a year to write your notes.

Within a month of your special day, you should have stationery, stamps, and addresses ready to go. Sit down in a quiet space and write each gift giver a thoughtful, grateful note. There is no substitute for this. Write five at a time, or ten, and then drop that batch in the mail. Work through your list until you have properly thanked everyone.

(this tip from Jennifer Manske Fenske)