I put up this picture of Boys over Flowers for two reasons:

#1 Because they had speech bubbles

#2 Because Boys over Flowers is just too wickedly awesome to ignore

Do's and Don'ts

DO'S
-Keep the dialog true to the character.

-Show emotion through action in place of the dialog tage.
   i.e. "I love you, Taemin." Sara studied the ground as she drew a circle in the sand with her big toe. OR "I love you, Taemin." Sara pounded his chest with her clinched fists and dared him to look away (creepy, Sara, reeeeeeeeeeaaaaally creepy~ I'd run if I was Taemin at that point xD). OR "I love you, Taemin." Sara covered her face to hide her tears. OR "I love you, Taemin." Sara pinched his cheek like he was five and walked away,

-Use dialog to move the story forward. Dialog must have a purpose.

-Use dialog to create conflict and build tension.

-Use dialog to break up description.

DON'TS
-Don't add emotion to your tag line using "ly" adverbs.
    i.e. "I love you, Taemin," she said nervously.

-Don't dialog the mundane.
    i.e. "Hi, Sook Hee," he said. "Hi, Jinyoung," she replied.

-Don't feel like dialog needs to be written in complete sentences.

-Be careful with hesitation words like "Uh" or "Um" or "Hmm" or really any other sound effect. Sometimes these can work, but if used in excess, they are distracting and slow the pace. You can create the feeling of hesitation or thought through action instead.

Other good things to consider as you write dialog:
-Speech can be messy. We ask questions answered with questions and talk over each other. Authentic dialog incorporates similar elements.

-Remember that less is almost always more.