The text-messaging age has slowly drained the professionalism and clarity from digital communication these days. The informal nature of instant messaging and email between relatives and friends has caused the formal email as we know it to die a quiet and forgettable death.
Though you may be friendly to your professors, it may not be ideal to email them the same way you might email your cousin Mandy about the boy you met last week at the football game. The professor might have trouble deciphering the lol's and omg's that riddle the message, and that same professor may not even give a hoot about how that boy looks just like Owen Wilson.
Writing an email is not unlike writing a paper: you want to stick the point and make said point in as few words as possible. Pay attention to the language you use when writing an email. Obviously an email riddled with four-letter words will not put you on the professor's good side, so do try to be as polite as possible.
Most importantly, however, "think about what you're saying," as Eastern Illinois University's Professor of English Michael Leddy writes. Unless it's down to the wire, you might want to give it some time before you email a professor regarding the unfortunate grade you received on your presentation. This is not to say that when you write an email to a professor that you should take the time to plan and draft as you would a paper, but you should do everything in your power to ensure clarity and conciseness that will allow your professor to read and respond to the multitude of emails they receive on a daily basis.
Professor Leddy created a blog post back in 2005 in order to solve the growing epidemic of poorly-executed student-to-professor emails. In it are things to pull from to use outside of the academic world as well.