Wedding thank-you notes should be hand-written, include acknowledgement of the individual or organization you are thanking, specify precisely what you are thanking them for, express genuine gratitude for their help, gift or attendance, and be sent as soon as possible after the wedding. Thank-you notes aren't just for guests and gifts, but should be sent to event space managers, caterers, planners and the like as well.
Timeliness is crucial. EmilyPost.com states that contrary to popular belief, married couples do not have a year in which to send thank-you letters out to people. Start writing them the first day time is available, and set a goal of doing a set amount every day until the task is done.
Hand-write notes on appropriate cards or stationery. Write on cards directly rather than including a separate note. Personalize your thank-yous rather than using a text template.
Specify what people are being thanked for, be it their attendance, a gift or some assistance. Name the gifts received precisely. It may seem rude to specify dollar amounts for cash gifts received, but it lets the giver know that the entirety of their gift was received. Be genuine and appropriately grateful, but do not be gushy.