Arrive early. Always, always, always arrive early if possible.
If not, arrive on time. Things go wrong, things need to be reworked. Other people run late. Bring coffee and a big bottle of water, and arrive early any time you can. (Also, the more responsible and reliable you are, the more you’ll get called back and the more you’ll get booked, the more you’ll get great recommendations to other photographers, and the bigger the possibility you’ll be called to fill in or asked to stay in the event another model doesn’t show.)
Text, call, or email before you are leaving (especially if you are travelling a great distance) so that the photographer knows you will be on time (or how late to expect you if something came up), and can plan accordingly. This is especially important if there are other models present to accomodate or when a hair and makeup team needs a schedule of who to get ready in what order. If multiple models or portraits are getting shot in one day, this also takes into account what order the shoot goes in, and if there are pair or group shots, who is and isn’t included, and when those can be completed in the day’s schedule.
STRETCH. It doesn’t matter if you do it at the shoot site or at home if you live close by. I can’t stress this one enough. STRETCH. You are going to be twisting, turning, and holding poses for an hour or more that your body isn’t used to doing. No one will think you look silly or weird. Anyone who knows what great portraits take understand it takes a surprisingly physical toll on the body. I often feel like I did yoga all day long, and on day-long shoots as a model, felt like I could justify skipping all the rest of the week’s exercise (I didn’t, but it felt like I could, considering how achy I was by the next morning).
Bring a snack and something to drink.
Also, make sure you have printed or handwritten directions and the photographer’s contact information. It’s embarrassing how many times unexpected construction will make you late or how often your GPS will not take you to the right location.