Question: How do I keep my dog out of the cat's litterbox?
Answer: If you're both a "cat person" and a "dog person", chances are you've run into this before. Litterbox "crunchies" are just so very appetizing to a dog! It's all that protein in cat food. Since cats are obligate carnivores (they must have meat, all meat), the waste product (also all meat) is very appealing to a dog.
Great! How do I stop it then?
The first and most obvious option is placing the litterbox out of reach of your dog. This can get very high up, especially if you have a Great Dane. It's a good thing cats are so agile. Put the litterbox up on a shelf, on a washing machine, or countertop (I know, that last one is a little odd. "What's for dinner Mom?", "Oh whatever's on the counter ...").
Other options include using a covered litterbox, or self-scooping types. Feel free to browse Franny's selection of litterboxes for more ideas on the About Cats site.
Of course, the best method, aside from removal of temptation, is simply training your dog that the litterbox is off-limits. Redirect his attention to an appropriate toy whenever he heads for a "kitty-crunchy" and praise heavily when he veers away. An extra incentive of a meaty treat would be a marvelous and more tempting reward for ignoring the litterbox as well.