Make sure that you have an experienced/organized wedding director. Most churches require that you use their person, but if you are not getting married in a church it's really important to have someone professional take on this role.
Make sure you inform everyone attending the wedding rehearsal of the time and place at least a week in advance and then continue to remind them as you see or speak to them, and advise them not to be late. Who wants to be stuck rehearsing for 2 or more hours. By the way, if you're the bride or groom, you must be on time!
I have learned that clear communication makes a wedding rehearsal successful.
One last tid bit . . . the purpose of the rehearsal is for everyone to know there places for the wedding ceremony, but more importantly it's so the Bride and Groom feel confident that everything will go as planned on their wedding day. If your director, does not make you, the Bride, feel confident and important, then get a new one, if there's time.
May you feel ever confident after your wedding rehearsal and may your wedding day be blessed.