Thankfully, we don't have to worry too much about the vocative case in English, because our nouns don't change depending on what prepositional jigger they're paired up with. It's still there, though, even if we can't see it, and one thing we have to remember to do is use commas to set off any noun that we're addressing directly. This can be a person, as in the following example:
"I really think you should read more prose, Glenn, because Crime and Punishment isn't a fucking poem."
Or it can be an object:
"How do you feel about being sat on by that morbidly obese woman, chair?"
Or a pair of abstractions:
"You're a painted whore, Justice, and you, Truth, are a metaphysical chimera."
You should also use commas to set off the construction you x when you're calling someone a name:
"You keyed my car, you piece of shit."
In old school English (which, if it's recognizable at all, is probably Early Modern English), like in the King James Bible, the vocative case is sometimes marked with an O, as in the following sentence:
"O God, thank you for creating Pan's Labyrinth, the best movie ever."
(This is not to be confused with the interjection "Oh!" as in, "Oh! Pan's Labyrinth was such a good movie that my balls are still tingling!")
In some other languages, like Czech, for example, it's a little more complicated, because the ending of the noun changes as well. My friend's name is Ondra, but I have to change it to Ondro in the following sentence:
"You're still my friend, Ondro, even though you screwed my girlfriend after you both got drunk at Skleněná Louka that time."
Some people lament the abuse of commas. Well, I say, "Don't forget--neglect is abuse too!"
(Image from www.realtorwives.blogspot.com.)