Assuming your friend is talented and you are not simply in the thrall of some deluded, pint-sized would-be diva, there are a number of things you can do to try to render yourself memorable.
1) Support everything she does and agree with everything she says. Famous people don't like downers. By the time she becomes successful, she'll be so used to having you around to prop up her unstable sense of self she won't be able to forget you if she tried.
2) Try to become the one person who knows her most embarrassing secret. Compromising secrets aren't all that compromising anymore (you'll also learn to adopt a cynical view of fame), so embarrassing is the way to go. Coke habit at ten years of age? Who hasn't had a tween stint in rehab? That's nothing. Become the only person to have documented your songbird's problem with flatulence and you'll have a friend for life.
Of course, it's altogether more likely that your friend will grow up to be a mediocre singer with a crumbling marriage and a dependence on whatever drug she was prescribed after that operation she had. She won't ever forget you because you're the one person who reminds her of her early promise and turned out to be even less successful than she is.