I've just been reminded of Deb's thread a while back about the clichés in a 15yr old narrator's voice, by stumbling on a classic Nicola Morgan post.
On that thread of Deb's I was talking about trying to find ways to make things fresh and vivid (i.e. not clichéd) while staying true to the child-sounding voice.
Nicola's post is about similes and metaphors, and is very good on thinking about suiting the connotations of your image to the narrator, as well as the denotation:
http://helpineedapublisher.blogspot.co.uk/2010/10/as-pointless-as-pointless-thing-care.html
What made me think of Deb's thread was the Chris Cleave example about halfway through.
And you might want a coffee alert.
<Added>Deb's thread is here:
http://www.writewords.org.uk/forum/65_443276.asp