Make sure she's eating canned food. Canned food contains lots of moisture, and water is what will help keep her system moving efficiently. Even so, if you can get away with it, sneak a bit more water into the food.
You can add a little olive oil, about 1/4 teaspoon, mixed in to each meal. Or try powdered psylllium husks (available at health food stores) mixed with some spring water to form a thick liquid, and added to each meal. It's very effective.
Pureed canned pumpkin (plain, no added spices) is usually tolerated well by most cats when mixed into food. It contains lots of fiber. If your kitty is prone to this problem, try adding a small amount of veggies like pureed or grated zucchini, green peas, grated raw carrots to her food several times a week. Small amounts of veggies (about 1/4 to 1/2 teaspoon) are fine for cats and add fiber to help avoid constipation.