Don't make your characters talk like characters do in real life. In real life, people don't have straight, smooth-flowing conversations that are designed to move a story forward. A conversation between two people meeting at the mall, for example, is apt to include statements repeated over and over again. There will be interruptions, subjects started and then dropped in favor of something else. Take this type of dialogue out of your writing. Conversation needs to lead the story somewhere. It should highlight an important plot element or uncover some bit of back story.
Be careful not to make the conversation between your characters too contrived. Some amateur writers like to have one character say the name of the other character they are speaking to in every sentence. An example of this would be: "So, Michael, how are you today?" "Well, Bob, I'm quite fine, thank you." This type of dialogue isn't natural. A better way to write this dialogue might be: "How's it going?" "Pretty good, thanks." In a story, you would want to avoid using meaningless filler dialogue like this too often.
Use selective phrasing for characters who speak with a heavy accent or a local dialect. Mark Twain was skilled with dialect writing. One of your characters might have a thick southern drawl. This doesn't mean you should try to spell out the dialect on every word she speaks. A few select words will be enough to get the point across, especially when your readers know that the character in question is from Georgia.
Don't have characters stating obvious information in dialogue. Use dialogue to impart information to your readers, but don't do it in such a way that the information seems forced. New writers sometimes make one character tell another character about something she already knows just so that information can be delivered to the reader. This is distracting, doesn't sound natural and should be avoided. If you have to relay information to your readers, find a way to do it naturally through dialogue.
Be careful with creative dialogue attributions. "He said," "she said" is most often sufficient. New writers believe this dialogue attribution is boring and they tried to spice it up with attributions like: "Yesterday was so awesome," he smiled. Using "smiled" as an attribution like this is clumsy and makes your writing weak. If you must use "smiled," write something like "he said with a smile." The truth is, most readers skim over the "he said," "she said," without a second glance. They expect it there. It doesn't detract from the dialogue like clumsy attributions do.