(From DevianArt, user: ShadowDeidara, full credit goes to him/her):

How to annoy the cast of Death Note

MISA:
*Steal her phone and change the names of the people in her contacts list to Harry Potter characters.
*Cling to Light.
*Randomly smack her in the head.
*When she walks into the room, look at her in disgust and say, "Ugh! It's a man?!!"

LIGHT:
*Tell him that he's not nearly as cool as L, and that he never will be.
*Tell him that he gets owned by an 18 year old kid who plays with toys.
*Ask him if he's gotten laid yet.
*If he says no, ask why not.
*Stare blankly at the back of his head. When he turns around, look behind you.
*Steal his Death Note.

L:
*Steal his candy.
*Repeat everything he says.
*Narrate his life.
*Stare blankly at him. Never break eye contact.
*Sing the song that never ends.
*RANDOM DANCING.

MELLO:
*Steal his chocolate.
*Blame it on your imaginary friend. Make it obvious that it was you.
*Draw on his face when he's sleeping (he could really use a nice mustache...)
*Steal random articles of clothing from him.
*While he sleeps, subliminally teach him Spanish.
*Sing the song that never ends. Get Matt to join in.
*Tell him that Near wins in the end.

MATT:
*Convince him that Pokemon are real, and that Mello is a Pikachu.
*Give him Nicorette for Christmas. Fill his stocking with it.
*Mix up his clothes. For instance, put random socks you stole from Mello in his shirt drawer.
*When he walks into a room, pretend that you can't see or hear him. Actually, this works for everyone.
*Dress as Mario and ride around in a go-cart screaming things like, "GET THAT MUSHROOM!"
*Steal his Game Boy.
*Cancel his WoW account.

MIKAMI:
*Tell him that his god is going to get owned by some kid who still plays with toys.
*Randomly scream at him.
*Run up to him, yell "DELETE!", and smack him on the head.
*Convince him that seals are sacred animals. Insist that he refer to them as "The Great Ones".

BEYOND BIRTHDAY:
*Refer to him as "Munchkin".
*Replace his strawberry jam with grape jam. See how long it takes him to notice.
*Scream at him about wearing too much makeup.
*Tell him to "stand up straight, dammit". This also works well on L.
*Tell him that you know when he's going to die.
*Dress (convincingly) as L, then go up and slap him. Run away screaming hysterically.

AIZAWA:
*Tell him that his hair is smiling at you, and that it's starting to creep you out.
*Ask him if he's from New York.

MATSUDA:
*Act normal around him, but as he's walking away, loudly whisper, "Useless."

SOICHIRO:
*Tell him that he gets owned by a Pikachu.
*Ask him why his mustache is so awesome.

MOGI:
*Tell him that he needs to smile more.
*When eating at McDonald's, insist that he orders a Happy Meal, and that he gives you the toy.
*Take him to see Alvin and the Chipmunks.

Lastly, ruin the ending of Death Note for them.

Warning: annoying Light will result in death. Highly not recommended.