Be genuinely interested in the girl and care about what she has to say. It's easy to tell when a conversation partner actually wants to hear you, or is just waiting for their turn to talk. Listen to her, ask questions about the things she mentions, take conversation cues from her (which mean that if she's giving you signals that she isn't interested in talking, don't try to force the conversation on longer - that will only give her a negative impression of you). Remember, you're not just trying to convince her to like you - you're trying to get to know *her* too! If you don't actually want to get to know her, then why bother talking in the first place? Conversation and friendship flows in two directions, after all.