Put sea shells or decorative rocks on top of the potting media. She
won't want to dig through those things. Also, make sure that you scoop
her litter box every day--a cat wont' use a smelly litter box.

Perhaps your cat doesn't like the (smell or texture) of the litter you
currently use...there are several litters designed to get around this
problem. My answer was Feline Pine..the only litter one of my cats will
tolerate (other cat only uses newspaper in his litterbox).  Is it dirt
your cat likes?  Perhaps sprinkling a little dirt in the litter will
help (decreasing the amount of dirt asap).  Is the cat's box dirty?
Some cats won't go in a dirty box.  Good luck.