The best piece of advice I can give you is be prepared to take lots of photos. This means having a large card in your camera so you will be able to take at least a few hundred photos. You may not use it up, but it helps psychologically to have a cushion. If you don't have a large card, you may want to plan breaks so you can load images onto your computer.
You will want your subject to be as well rested as possible for the session. You should schedule your session at a time that isn't too late in the day so she is worn our, nor do you want to schedule too earlier because, well, most of us need an hour or two to look our best.
I suggest having a lint brush on hand to take care of any specks or flecks that might be on your subject's clothes. Her clothes should be as ready and wrinkle-free as possible.
If you are using a backdrop, make sure it is clean and also wrinkle-free. (For more on this, visit the section on backdrop trouble.)
Finally, you may want to put together a shot list — that is a list of poses and/or photos you would like to try. You don't need to box yourself into a specific set of photographs, but having a list to work from may help keep you focused and keep the photo session flowing.