If you get to cut the hair, decide if you want to keep it in a ponytail or braid. It's much easier to do a braid before the hair is cut. A braid has the advantage of being less likely to lose loose strands, but it's also more likely to become unkempt over time. A braid can also be tightly secured at both ends, which helps. If you don't know how to do a neat tight braid, consider having the model do it--she certainly will know how. If it's a very long ponytail that's not to be braided, consider tying it at multiple places along the length, like in:

For ties, consider using the old-style hair elastics (the ones that feel like thin bungee cords, not the modern snag-free scrunchies which are too loose) bound very tightly. Rubber bands would be fine except that they degrade over time. Zip ties are effective, but IMO give it too much of an "industrial" look. Thick string-like cords (not kite string, but not twine either) or ribbon, as in the picture. are also effective. If the hair has to be cut in sections, you can also use string to tie it into a ponytail/bundle; lay the cut hair perpendicular to several pieces of string and tie off when done.

Cutting (or even being present during cutting) of a woman's long hair is a bittersweet but amazing experience that many of us will get to experience either never or only once or twice. Remember to be gentle and reassuring with the model, and to tastefully compliment both her old and new looks; she is literally letting you strip her of what is likely her best feature, and is likely quite anxious. Good luck.