What do you do when your cat won't listen to you?

I'm pretty sure that refusing to obey is second nature to cats and I've often heard that they can't really be trained. We have fun joking about our cat because we can only give the *illusion* of obedience. Like if she's plopped down in the middle of the floor, my daughter says "sit!" and then "good girl" as if she wasn't already sitting. :-) Or if we need to step over her, we sometimes say "don't move... good girl!" Because fooling ourselves about that is the only way to think that she'll listen to us!

I've also had a friend laugh at me for trying to get the cat to do something I wanted her to do. The friend told me that the cat is surely saying "You want someone to obey you? Get a DOG!" :-)