There are a few things I would check before the actual auditions too:
Does the director require a photograph of each applicant (Some do, some don't, more often they fall in the later). If they do, you need to advertise that fact, and you need to have a camera on hand to take them if people have forgotten.
You need a nice big table for people to fill in forms on, and you need a nice big table for the pannel.
If it is a musical, you need a pianist and a tape/cd player
Have plenty of water lying arround - both in the waiting room and in the audition room.
Schedule breaks. Even if more people turn up than you expect, make sure that the pannel of selectors get a break, and that you yourself take one.
Find out if they want the whole thing taped. It is a royal pain in the rear when they do, but sometimes it makes life easier down the road. Usually I would not suggest taping at an open audition, but on callbacks, certainly.
Find out if the pannel will be conducting callbacks today, or later on, and if they wish to advise the auditionees of their callback immediatly, or if they wish to go through a short listing process and for you to call them later.
--Outline of an average (non union) open audition for me--
Auditions open at 9am. I arrive at 7:30/8am
I ensure I have the following:
Lots of pens
Stapler
Paper
Black markers
Cups and water jugs
Cd player
Spike tape (mainly if recording)
Audition registration forms
Audition evaluation forms
2 tressel tables
Chairs
2 outbox/inbox type trays
The waiting room: Tressel table along one wall, with a sign on it saying "REGISTRATION". I will usually make a few copies of the script, or at least excerpts of the script available, as well as placing a pile of pens and audition registration forms there. I usually like to have someone behind the table to accept registrations and place them in a pile, ordered by who submitted first. I also make sure that there is access to plenty of water - I have a small water cooler (about 4 litres) which I bring, and a stack of plastic cups.
The audition room: Tressel set up at the back of the room with chairs for yourself and members of the panel. At each place (Maybe your own, maybe not, ask the director) put a pen, and an audition evaluation form. At your place, put the stapler and the two 'outbox/inbox type' trays. Place a couple of jugs of water on the table and cups at each location. Spike a 'space' out for them - either an X or a space, and place a chair. Place a sign on the door "Knock and wait for admitance"
How the day runs: Doors open, people register, person behind the desk calls out the first name, hands them their form and they walk into the audition room, handing the form to you. The audition is run, you gather the forms and staple them behind the registration form, and each member of the panel gives a yay or nay. Now depending on what you agree on before hand, often if it is the majority are nay, they go into one tray, otherwise they go into the other, sometimes it would only be if all are nay. Sometimes, they dont want them sorted out. Anyway, whilst this is happening, outside, the 'registra' has told the second person to go through, and you can call for them to enter, and the thing repeats. ALL DAY. with a break for lunch of course.