You can't always get your Mom to say "yes." But, you can present your case very well. For example, give her plenty of information, like if your friend's mom will be there for the sleepover. You can also suggest she call your friend's mom for confirmation. The more information you give, and in a calm manner, the more likely you are to have a productive conversation. Try not to get angry if she says "no." She may say "yes" next time if are mature about a "no."