I had a client whose cat began meowing alot in a way she hadn't before. The meowing got increasingly more frequent. She had physically examined her cat but could see nothing wrong. I convinced her to go to her veterinarian and sure enough a small foxtail had been working it's way into her cat's skin between the toes. Once removed, her cat returned to her normal "just a little meowing" self. If you have eliminated any health concerns, let's look at your cat's social and territorial situation.
If you have moved, gotten another cat or other pet, moved the couch or changed your work schedule, meowing can start and sometimes become a habit. Another cat coming around your house will set off meowing as well. Read the article on cat territorial behavior for complete details.
Your cat may have learned that all she has to do is meow and she gets whatever she wants - food, attention, affection. Often, what starts out as a demand for attention soon becomes a self-reinforcing habit. Now kitty will meow all day just for the fun of it. This situation becomes even worse if the cat is lonely or bored.
The first step in breaking this habit is to exercise your cat daily with active play sessions. Don't expect kitty to exercise herself. Many cats will swat at a ping-pong ball or a catnip mouse once or twice then lose interest. You must get involved in the game. Tie a string to the mouse and pull it slowly, temptingly and repeatedly across the floor. Give kitty a good workout - expend some of her energy.
Pay attention to kitty and give her what ever she wants, but ONLY when she is quiet. Ignore her whenever she begins her vocal blackmail. Don't give in. Each time you give in to your cat's verbal demands, you are teaching her to meow even more. If you wait until your cat is quiet, she will soon learn to associate silence with rewards. Some cats enjoy talking and some owners enjoy their cat's chatting to them. But if you want a few moments of peace, you can teach your cat to be quiet on request. Gently ask kitty to "quiet." If the cat ignores you, immediately shout, "BE QUIET!" or clap your hands loudly. After a few repetitions, kitty will get the idea and obey the gentle request of "quiet,"rather than get screamed at or startled.