The best GTD tip one can give you is not on Lifehacker, not on 43 Folders, and not on The Simple Dollar.
Its not about Zero Inbox, Clear, Remember The Milk, Things, OmniFocus or a myriad of tools. Its just about one thing:
Do any task as soon as it comes up. Without delay. Take the first step towards completing the task, because that is always the hardest. Everything after that is cause and effect.