Some sites with useful information on the subject of inappropriate elimination follow. They should be required reading for all cat owners. Understanding inappropriate urination even before it occurs is often helpful and will make you aware of factors to avoid or change to prevent a problem in the first place. Be sure the litter box locations are private but accessible and not in high traffic or noisy locations. Clean the liter boxes daily.
Cats are fastidious and many simply refuse to use a dirty box.
Remember that the likelihood of litter box lapse increases dramatically with increasing cat numbers in your house. Seriously consider each addition before adding cats to an already overcrowded household. Cats have litter type preferences. Sometimes you have to use trial and error to discover which your cat likes best. Don't change types abruptly or frequently once you find your pet's preference.