How do you ask people to model?

[meat]

Very politely. And you don't have to explain what the pose is for exactly. Could be for studies, help you hone your artistic skills. If they ask to see it when it's finished, and you really don't want to, you can put on a gloomy, distant expression, explain it gave you a very hard time, and the inspiration died, and look depressed. They won't press to see it after that.