There are several ways to avoid going away from keyboard. The most popular is to get a macro that does a lot of movement, but macros are limited and only run for so long. One way is to use the built in windows ease of access controls. You can set ease of access up to remember keystrokes and then execute them. If done right you can have windows keep you from going AFK indefinitely. There are add-ons from curse that will help you with this as well (UPDATE: The add-ons are no longer available. )
There aren't a lot of reasons to avoid AFK unless you're worried about mana biscuits disappearing. This used to be helpful in battlegrounds to farm honor but now you get reported by other players for being AFK before the round ends which means you never receive honor. A lot of people use that search to arrive here, I figured giving them what they're looking for would be nice.
