How do I keep her instrested in me?

Well I'm guessing she's interested in you at the moment, thus why you said "keep her interested". Well you don't want to keep the girl waiting long; as much as they like a little playful teasing, they don't appreciate a delayed flight. Spend more time with her, talk about her, show that you're interested in her and give her a reason to feel good about herself and she'll be interested in you. But you also want to close the gap before she loses you. As hammy as it is, people do actually wake up in the morning with different feelings. Ask her out, the worse that can happen is she says no. Good luck :)