When you have a booking you want to make sure you are prepared. This can mean prepared as in knowing all the details, and bringing things to the job which will help you manage. Your agent should help you with this. However here is insight into how you can prepare for a booking:
Shoes. You should bring a comfortable pair of shoes to wear during breaks, lunch, etc.
A robe. If you have multiple changes you should bring a robe to wear inbetween them and also for body part modeling jobs you should bring one with you to wear before you get on set.
A snack. And water. Bring some fruit, or peanuts, or something incase you are not given lunch like you thought or the day is cut short but you still might be hungry. Also you will want energy for the job, so bring water!
Hairclips and hair pins, etc. Since many jobs ask you to bring your hair natural, you might want to bring your own comb or brush and hair pins, especially since some jobs might involve pinning your hair back and sometimes the stylist doesn't do everything for you.
A cell phone charger. Even though you are booked and can't be yappin on your phone, you will get breaks, and you can check your voicemail, but being prepared can mean sometimes bringing a phone charger if you feel you might need to check messages or are low on battery.
A Book or magazine. You will want something to do during your breaks, or before the shoot,so it is smart to bring a magazine or book.