Here's a list that I came across. What are your favorites? Add your own! Note: I didn't write this list so I apologize for any misspellings or grammatical errors.
We may not be in Professor Flitwick's class, but you still are charming.
I don't have an invisibility cloak but do you think tonight I can visit your restricted section?
I want to get my basilisk into your chamber of secrets.
My love for you burns like a dying phoenix.
You know, Hagrid's not the only giant on campus.
Do you think you could take on a mountain troll in the bathroom like Hermione did? Would you be ready in five minutes?
Wanna make some magic together? My wand is at the ready.
If you were a quaffle and I was a chaser during a quidditch match, I'd score with you.
I must have had some Felix Felicis because I think I'm about to get lucky.
Without you I feel like I'm in Azkaban and dementors are sucking away my soul.
You know, the Sorting Hat placed me in Gryffindor. I think it's because like Godric Gryffindor himself, I too have an impressive sword.
Want to go to the Hog's Head, if you know what I mean?
Being without you is like being under the Cruciatus Curse.
Want to have a Tri-Wizards Tournament? Well not really "Tri-Wizard," I was thinking more one wizard and two witches.
I've been whomping my willow thinking about you.
If I was to look into the Mirror of Erised, I would see the two of us together.
Do you know the Petrificus Totalus spell? Because you make me stiff.
I'm not an Animagus but sometimes I can be real animal.
How 'bout you and me go look for the Room of Requirement?
You know Platform 9 and 3/4? Well I know something else with the same exact measurements.
A couple nights with me and Moaning Myrtle will have to get a new nickname.
What do you say we disapparate out of here.
Wanna practice making what looks like a mandrake.
- You look like you'd be a good Quidditch player. Want to ride my broomstick?
- You know, when I said, "Accio hottie," I didn't expect it to work!
- I know my name's not Luna, but I sure know how to Lovegood if you know what I mean.
- If I try hard enough, I can get a really big patronus. All I have to do is think of happy things.
- If I were going to produce a patronus, you'd be my happy thought.
- Will you be my horcrux tonight, so l can give a piece of my soul to you?
- I know you want me to manage your mischief!
- You must not be a muggle, because you cast a spell on me.
- You must be magical because I've fallen under your spell.
- Want to head to the Shrieking Shack? We could do some shrieking of our own.
- You don't need defense against my dark arts.
- Are you using the Confundus charm or are you just naturally mind blowing?
- The thought of you makes something vast and silver erupt from my wand.
- I must need Occlumency, because I can't get you out of my thoughts.
- I might as well be under the Imperius curse, because I'd do anything for you.
- (for the females) Is that a wand in your pocket, or are you just happy to see me?
- If I were a Seeker and you were a Snitch, would you let me catch you?
- I don't have any muggle money, but I do have a sickle and two knuts.
- Are you a bogart? 'Cause I have a fear of hot girls.
- After a romantic night with me, you are going to need a timeturner because you are going to want to experience it again and again.
- Are you speaking parseltongue? 'Cause you're talking to my snake.
- Your smile's like expelliarmus: simple but disarming.
- They say I'm like the horn of a crumple-horned snorkack. Explosive.
- What's the password to your portrait?
- My heart's splinched without you.
- Yeah, I've got a pretty good batch of polyjuice going, wanna taste?
- I'm goblin. Let me stroke your vault.
- You're like a bottle of Skele-Gro: growing me a bone.
- Why dont i make like Salazar and Slyther inside of you?
- Your name must be Severus Severus 'cause you make my prince full blood.
- Did you survive Avada Kedavra? 'Cause you're drop dead gorgeous.
- What flavor are your Bertie Bott's? (???????????????)
- I can be your house elf. I'll do whatever you want and I don't need any clothes.
- I need a pensieve because my head is filled with thoughts about you.
- How do I get into your Department of Mysteries?
- Why don't you come tame my dragon?
- Just like the Slytherin common room, how 'bout my snake in your dungeons?
- I don't know a thing about Merlin's pants, but I'd love to get into yours!
- Wanna play with my Sorcerer's Stones?
- You don't have to worry about me, I've been tested for Hogwarts, if you know what I mean.
- If you were a basilisk, I wouldnt mind dying just to look into your eyes.
- Are you a dementor? Because you send chills up my spine.
- I'm just like Oliver Wood, baby... I'm a keeper!
- You might be a *insert quidditch position here* but I think you're a keeper. (Take your pick.)
- Engorgio! Oh wait I don't need magic to enlarge this!
- Firenze and I have a lot in common, we both have parts like a horse.
- My name isn't Neville, but i do have a longbottom. (I just thought this one was ridiculous which made it funny.)
- Did you say "Wingardium Leviosa"? Cause you've got me rising, baby.
- So your the Head Girl of your house, huh? hmm.....
- Cho Chang? More like Cha-Ching! Cause I just hit the jackpot.
- Oh, no everything’s cool. I got the Fiendfyre treatment, no more crabbes.
- Could I borrow your wand? I need to practice my 'swish and flick.'
- Are you a dementor? Because you just took my breath away.
- Yeah girls call me "Aguamenti." Everytime they hear my name, they get wet.
- It's a portkey.... once you touch it, it will take you somewhere you ain't never been before.
- I solemnly swear I am up to no good...
- Want to learn to speak troll? I can get you grunting in no time.
- Would you like a butterbeer? It's a portkey. Next thing you know we'll be back at my place.
- I don't need the mirror of Erised to know that you're everything I desire. (similar to a previous line, but better I think.)
- I'm like the spine on a care of magical creatures book; if you stroke me right i'll open wide for you. (Pretty much a witch's line)
- You know...I have been mistaken for a centaur before. (similar to a previous line, and a bit
- Did you just cast a spell on me baby? 'cuz I'm feeling an engorgio charm coming on.
- I know you're taken, but if I had a time-turner, you'd be mine. (How 'bout that one, huh?)
- Is your name "Avada Kedvra"? 'Cause you've got a killer bod.
- The sorting hat says you should be in my house...wait ...whats that...it also says you should be in my bed.
- Did you use Relashio? 'Cause there's sparks between us.
- Did you slip some firewhiskey into my drink, or are you just getting hotter?
- Did you cast Impervio on me? Cause when I'm near you I can't control my body.
- Not even Veritaserum could make me express how much I’m truly attracted to you.
- If you were a Dementor I would turn criminal just to get your kiss.
- Do you want my Elder Wand, because you're looking Deathly Hollow...
- You don't have to say "Luminos Maxima" to turn me on.