As the guru himself (David Allen) suggests, getting to "IN" is the first natural step in putting together a solid Getting Things Done system. Here's the idea: Collect everything in your life (business or personal or both.....your call) and put it in a pile. If an item is physically too large to put in a pile, write a note about it and put the note in the pile. When you're done you will have collected all the things you need to do (or maybe not do) into one big, sexy pile. Easy, right?
That's just the first step but it's a liberating one. Staring down that pile and tearing through a handful of things everyday really makes for a cleaner, neater, more efficient brain. Trust me.
Now, before you go collecting things into piles keep this in mind: After you have collected all your stuff and are ready to tear through things to get to the bottom of the pile ask yourself a question about each item. Is there an action associated with this item? If the answer is "no" then do us all a favor and throw it away. You're not going to do anything about it and the collection of dust on this thing is probably already pretty impressive.