Don't try to be the Band Manager. Stay out of the business side of things.
Never ask bandmates for relationship advice.
It's not mandatory to attend every gig of your boyfriend/girlfriend's band.
Don't complain when your "special day" gets canceled because a gig is booked.
Don't get jealous when your boyfriend/girlfriend talks to groupies. They are the ones buying the merchandise and paying to get in the door. Let them have their moment.