Tried and true formula for unrequited puppy love with the best of intentions:
Flash 'em the biggest, warmest smile you can, look at their eyes like their face is a puzzle and raise your eyebrows like the first thing they say to you is the biggest surprise you've ever run into. Laugh quietly and make a joke that implies you have nowhere better to be. Avert your gaze and politely dismiss a compliment, but don't disagree with it. Out of nowhere, stop smiling, avoid eye contact and ignore them like you've never seen them before. Never smile at them again.