I figured I had to compile them someday, just so then I'd know for sure what I've seen before and why it's so bleeding familiar. So yes, plot devices, you know, those elements to a plot? Well, mainly covered for manga here.

Shoujo

Hidden identity
eg. Dengeki Daisy

Mistaken gender/identity
eg. Ouran High School Host Club

Best Friends
There's a lot of them all right? Just go pick a random one.

"Love Experiment
eg. Watashi Ni XX Shinasai!

Illness/Psychological Condition
eg. Beast Master

Rivalry
eg. Special A, Skip Beat

"A Secret"
eg. Hana Kimi, Kaichou wa Maid-sama

Arranged Marriage
eg. Goong

Servant/Maid
eg. Kaichou wa Maid-sama

Outcast lead character/love interest
eg. Koko Ni Iru Yo

Unbelievably destructive girl
eg. Chrno Crusade

Unbelievably gluttonous girl
eg. Chrno Crusade

Unbelievably dense girl
eg. Chrno Crusade, Ouran High

Ice Prince/Princess
eg. Watashi Ni XX Shinasai!

Popular guy/girl
eg. Ouran High, Vampire Knight, Skip Beat, Hana Kimi, Hana Yori Dango, Goong

Rich guy
eg. Ouran High, Hana Yori Dango, Goong

"Trying to Help"/Guy with problems
eg. The One, Hana Kimi

Resemblance to relatives
eg. He's Dedicated to Roses

Imperfect/plain female lead
eg. Koko Ni Iru Yo!, Zettai Kareshi

Pick and mix, write a basic plot linking everything.
Ooh lala, a shoujo manga plot by yours truly~