I don't post stories personally, I don't think it's that effective. You can take the occasional mirror gym selfie or beach/pool selfie but it's unlikely to get you anywhere and more likely to make you look douchey.
My typical course of action is to message her immediately after she gives me her Snapchat username. If you were having a conversation, just continue it, if she sends it to you a day or more after you ask for it, then just start up a new conversation.
If we don't speak for a couple of days, I might reengage by sending her a snap or will wait for her to post a story and reply to that depending on how often she posts stories. Generally speaking though, the best time to reengage is when she posts a story of herself. When a girl does this, she is purposefully inviting male attention. She might even be posting it for you specifically so that you start a conversation with her, so don't overthink it too much