Talk about Lily at every opportunity and hand him tissues
Leave lilies on his desk from an anonymous sender
Carve a big heart with "LE+SS" onto every wooden object he owns
Ask Snape what the biggest fish he ever caught with his nose was
Ask him if the reason he never washes his hair is because Lily once touched it, hence he forever cherishes it. And he's thinking "OMG, she touched my hair, OMG, OMG, OMG..."
Tell him he'll most likely die an old virgin (sad but true)
Ask him what fast food restaurant hooks him up with all the grease he puts in his hair
Inform Snape Neville would like him to wear an ugly vulture hat and a drab dress
Ask Snape if he heard a mysterious ticking noise. When he says he might've heard something, tell him, "Um, Professor, hearing things isn't a good thing, even in the Wizarding World. You should get that checked out. I know this great shrink..."
Make random hissing noises to make him mistake you for a Parselmouth
Create a profile for him on eHarmony, a Muggle dating website
In Potions class, fling random ingredients itno your cauldron, boil it down, and say you're a non-conformist to restrictive potion-making
Stir clockwise when you're supposed to stir counter-clockwise. Sue me.
Tell the Daily Prophet he's an unregistered Animagus