Make sure she also knows how to stand tied, and be patient. If you need to hop off to "use the restroom/find a bush" on a longer ride, or just take a break at a rest stop, you'll want a horse that isn't going to be prancing all over the place.
Be sure your horse will do okay with other horses passing by on trail, either from behind or oncoming traffic.
She should be responsive to you - and you should know how she is inclined to react if she does spook. Some horses want to spook and bolt, others will shy suddenly (and drop a shoulder when they do), while still others may spook but hold their ground.
Get a lot of practice mounting and dismounting from the ground if you are used to using a mounting block. While I generally prefer to find something to mount from (like a log or rock or stump), sometimes there just isn't anything available. Also, try mounting/dismounting on your horse's "off-side."
Try riding with a rain slicker, or some other fabric draped over the horse's hindquarters. Make sure your horse is used to seeing you and other people wearing "flappy" clothing like an outback coat or poncho. Even simple things, like getting her used to the sight and sound of a map being opened (on the ground, and on her back) are helpful. You wouldn't believe how many people I've run into on trail whose horses freak out when the rider tries to open a trail map to check their position.