Let her boyfriend have sex with her.
Or in another words, not to let your boyfriend own/play before/gotten your body. Cause as you know, the same rules applies here!
Men craves for things that they couldn't or yearn to get, and once they have gotten it or own the thing, they will get sick or tired and take it for granted.
And sooner or later, he will leave you or find someone "fresher".
I'm not really sure about how true is this theory by my grandma, but come to think of it, I thought she do make some sense!
Thus it's pretty true to a certain extent. Since, she had more experience as compared to all of us in life. Well, at least to me.