I've always found that being direct as possible works well for me. Flirting and playful politeness is always a plus, but sometimes it's foolproof to just walk up to the little lady and say "this might sound crazy, but I can't stop looking at you and I decided you seem like someone I'd like to get to know. Would you like to do something with me sometime?" Girls these days are so used to subversive conversation and pickup lines that directness and honesty are generally like a breath of fresh air.