This article is from the General Cat Care FAQ, posted to rec.pets.cats newsgroup. Maintained by Cindy Tittle Moore with numerous contributions by others.
It is possible to train a cat to use the toilet rather than a litter box. One book is How to Toilet Train Your Cat: 21 days to a litter-free home by Paul Kunkel, published by Workman Publishing, 708 Broadway, New York, NY 10003, and simultaneously published in Canada by Thomas Allen and Son Publishing (no address given). ISBN no. 0-89480-828-1. Cost, $5.95.
The cat must be well trained to the litter box first. Move the litter box into the bathroom next to the toilet. Little by little (2 inches every two days) raise the litter box until the bottom of the litter box is at the level of the toilet (seat down, lid raised). Then slowly move the litter box over to the top of the toilet. This accustoms the cat to jumping UP to the toilet to eliminate. When the cat is comfortable with this, cover the toilet (under the seat) with strong plastic wrap like Saran wrap and fill the middle with litter. Decrease the amount of litter until the cat is peeing into the plastic and then make a hole in the middle of the plastic so the cat gets used to the sound of urine and stool hitting the water. Sooner or later you eliminate the plastic.
Placement of litter box
Beyond making the litter box readily accessible to your cat, there is some consideration as to an aesthetically pleasing placement. Utility closets that the cat can always access are useful. Laundry rooms work well, bathrooms less well (especially in guest bathrooms). One suggestion was to build a chest with an entrance at one end big enough to contain the cat box. The chest can be displayed like furniture and yet be discreet. If you can't build a chest yourself, it should be relatively easy to saw an opening in the side of a pre-made chest.