Geena Davis, in her current role as a generous well of irreverent Hollywood anecdotes, has an Effective Tip for you, should you find yourself hit upon by an actor whom you don't wish to sleep with. This is News You Can Use. And, gloriously, Dustin Hoffman invented it! Here's Davis, paraphrasing Hoffman:
Your co-stars are going to hit on you. You should not sleep with your co-stars. It's a bad idea. I just recommend completely against it. Here's what to say, when they hit on you: "I would love to," be very flattering, "but I'm afraid it would ruin the sexual tension between us."
She used it with Jack Nicholson! Hah!