I used a great line on a guy just yesterday. Well, I'd like to think it was a good line.
After an initial "feeling things out" phase, and on a third kinda-date (we're friends), I asked him: "What magic are you using? I couldn't stop thinking of you yesterday".
He was a bit speechless at first, and I thought there was an awkward moment (as I had said I only wanted to be friends). Then after the conversation had meandered through other topics, he suddenly asked "So what were you thinking about, when you were thinking about me?"
Hah! Caught him.