Here's a critique I've received more than once: "your character talks like a character from a book. He's too eloquent, nobody really talks like that, unless they grew up in a library."
Now, to some extent, characters not talking like we really do is an acceptable break from reality: interjections, pauses when we look for the right word etc. break the flow of the narrative, so they would only be used to that deliberate effect, and sparingly, or they get annoying. (See also TV Tropes: Realistic Diction is Unrealistic and discussion of the trope here)
However, the flip side of this coin is, if I have a modern teenager saying "I will not forsake the land of my forefathers", I'm breaking the readers' suspension of disbelief. My character might reasonably feel like this, but that's not how he would express his feelings. And the moment my character has become to my readers "a character from a book" rather than "a human", I've lost their sympathy for him.
How do I keep to the golden mean between those two problems? What techniques can I use to ensure my characters talk in a way that's neither too bookish, nor too literal? My goal is for the dialogue to feel natural (even though it really isn't - see links above), so readers listen to what is being said, while how it's being said becomes transparent, or else enhances the story - definitely doesn't distract. The question is particularly pertinent for main characters: side characters with little "screen time" can get away with verbal tics that would become annoying in the characters whose speech one has to follow all the time.