Don't acknowledge that you think she's out of your league.
If you want to ensure a gorgeous woman never sleeps with you, or even vaguely wants to hump you, admit to her that you think she's out of your league. Tell her she's way hotter than the women you usually sleep with, and repeatedly say, "I can't believe this is happening," while she is talking to you/dancing with you/on top of you.
The moment you've acknowledged a disparity exists between you is the moment you've lost her. You just called attention to two things: that you think you're undeserving, and that if she were to go out with you, she'd be dating down. In other words, you've just admitted you have low self-esteem, and told her she doesn't know her own worth—the definition of a fail.