Make sure kitty isn't eating garlic is a good first step while great for keeping fleas away makes for a stinky kitty. You can also get a covered little box and put regular old baking powder in the bottom with the litter over the top. Also a dryer sheet on top of the covered litter box can also help. And I'd call your vet too, I had three cats for the longest time and their litter was never horrible. I mean it was bad but then again it is poop.