Make sure your litter box is clean, and that your cat thinks it's clean. Cats want to be able to bury their waste, and if there's too much in the box for the cat's liking, they'll go somewhere else, and it's often right outside the box if there isn't something else they could use. It's important to understand that it's the cat's opinion that matters here, not yours: you may need to scoop it every day, even if there's only a little in it.

You may also need to move the litter box and clean the previous area, including and most importantly the place outside the litter box that gets used. Use vinegar if you can: it has a strong smell that cats don't like, but it won't hurt them like bleach can. Lemon juice works well for this, also. What this will do is make sure that this area doesn't smell like a place they have used as a litter box before.