Leave the music to the last minute. Creating a playlist takes a long time, and don’t leave it up to the band because they might have a whole different agenda.
Have too big a bridal party. Limit the number of bridesmaids to three or four or you will end up dealing with drama.
Let difficult in-laws break your spirit. Kill them with kindness and acquiesce to their requests where you can but stand your ground on the points you are really passionate about.
Underestimate how long it will take to finalise your seating plan for the reception. Leave lots of time to really think through the nuances of the relationships between your invitees.