Good advice from mojoe above.
The thing with dialog (as with everything else in a story) is that it must advance the plot or story or provide insight to the characters etc. It must have a reason for being there and being dialog instead of description or action.
Here are a couple of links:
http://fictionwriting.about.com/od/c...p/dialogue.htm
http://www.sfwriter.com/ow08.htm
http://www.right-writing.com/natural.html
http://www.musik-therapie.at/PederHi...gue&Detail.htm