After reading thousands of posts that don't even approach English, I decided to make this short and easy guide.  Hopefully, this will lead to improved posts in the future.

The There/Their/They're Distinction

There - This one refers to location, as in "The Xbox is over there." or "There are serious problems between EA and its employees."

Their - This refers to posession, as in "Microsoft has their sh*t together." or "Their Xbox Live service is awesome."

They're - This is a contraction meaning 'they are'.  You should use this only when the words 'they are' would make sense in its place.  "Sony rocks.  They're releasing the PS3 soon and it's gonna roxorz."

The Too/To/Two Distinction

Too - Refers to an excess amount.  "The PS3 might be too expensive for me."

To - Indicates direction.  "I'm going to E3.  w00t!1!"

Two - Is the number 2.  "Most people who post on Gamespot's forums have two brain cells."

The Its/It's and Your/You're Distinction

Its and Your - Indicate posession.  "The 360 can download demos and store them on its hard drive."  "May I please play your Xbox 360?"

It's and You're- 'It's' is a contraction for 'It is'.  'You're' is a contraction for 'You are'.  "It's a shame that Nintendo changed the Revolution's name to Wii."  "You're probably a hardcore gamer if you've scored a million points in Geometry Wars."

And there you go.  A couple of simple definitions to make your posting more readable.