Well, you can find a lot of general dos and don'ts all over gradcafe. Relating to earth sciences, I have always been told to not be very specific with what research you want to do. Obviously don't say something vague like "I want to study rocks", but don't be as specific as saying something like "I want to study the bedrock of the Quabbin Reservoir to determine the influence of aluminosilicates on chemical weathering over the last 100 years". I totally made that up, but I hope you get the idea.
As with any job application, you want to emphasize how good of a fit you are and how you plan to make great contributions to the scientific community. Don't focus all on you and how grad school will help you with your goals. Why should they want you?
Papers, posters, talks, etc. are really good, so don't forget to put them in your paper, maybe as segways to what you would like to study in the future. But don't keep talking about how awesome you think you are, and how smart you think you are, and how you've done all these amazing things, because it makes it seem like you think you're the shit and you already know everything there is to know about everything.
Good luck!