How do I tell my cat I don't want to play?

Okay, I love my kitty first of all, and she loves to play with me. But there are times when she wants to play when i"m just not int the mood for it. I know you can't train cats like you can dogs, but is there a polite way to tell her this. I have to push her off the couch to get her to leave me alone. Is there a trick or something?