While we are on the topic of litter boxes, make sure you have enough boxes for the number of cats you have. The usual rule of thumb is one for each cat, plus one. However, this is not always necessary. Make sure the litter box is in a convenient place for the cat to reach from anywhere in the home. Putting it too far away from where the cat usually hangs out is asking for accidents. Be sure you scoop the litter at about the same time every day. Your cats will know when it's time to scoop. They will race each other to use the fresh litter box. DO NOT scrub and bleach your box weekly or monthly. Cats feel more comfortable using a box that has their scent in it.
I recently purchased a huge litter box to put in a common area in the house. One cat laid in it for a couple of hours. The others raced to use it multiple times. It is now the litter box that is used most often! Good thing it's a BIG one!