Sign your card. Since formality is not a requirement for postcards, say goodbye like you were talking to them. When was the last time you said "sincerely yours" to somebody, face to face? Instead, there are the old standbys: "Wish you were here!" "See you soon!" "LOVE LOVE LOVE!!!!" Or, you can sign off in a personalized way. Examples are “Love, your wandering granddaughter” or “Signed, a broad abroad” or something pun-y like that.