Phrases like "I think of you as a friend" or "let's just be friends" is where the term "friendzone" comes from.
It's BS. A lot of self-proclaimed "nice guys" use the term in situations where they gave a woman something of value - be it time spent together, food, gifts, whatever - and think that means they're entitled to sex. Sometimes the term is used in frustration, because it sucks to have unrequited feelings for somebody, but that doesn't excuse the implications. Anybody genuinely expecting it to work like an exchange of goods/services needs to learn the difference between relationships and prostitution.
Not flirting-specific, but general conversation tip someone gave me: ask open-ended questions. If you ask the right questions, the other person will be the one talking. Example, if someone saw a movie, asking "was it any good?" is easily answered yes or no. Asking "What was it about?" or, "Who was in it?" invites them to talk about it, at which point you listen and pick up things you can comment on ("Oh, so-and-so, weren't they in [some other movie]?") or ask more about.