Sadie: Tug on her sleeve and bat your eyelashes while saying “Pleeeeeeeeeeeeeeeeeease!” And if that doesn’t work, make it a “Pretty Please!” 😉
No, really… there are so many things to consider about this: what mood they’re in, what mood you’re in, whether you’re both playing, or you’re trying to put out feelers to see if they want to or not, it all depends. Aside from all of that, the best way is to be straightforward and ask, while being polite.