Every so often literary agent, Nathan Bransford, posts advice on his blog that is just what you needed in your writing life. Last week he wrote about dialogue. In fact, that blog entry is called, Seven Keys to Writing Good Dialogue.
Click on the link and take notes. This stuff is good.
Every Friday he posts current news from the publishing world. If you only have time to read one of his blog entries, Friday is the one you should pick. It may take several hours to follow all the links.