The habit of doing something as soon as it shows up if it will take less than 2 minutes. The idea is that this is roughly the point at which it will cost more to manage the item in your system than to just do it.
The practice of keeping strict boundaries between items of different types. In GTD, for example, it is highly recommended to keep reference material (which has no emotional cost) separate from commitments and actions you need to keep (which have strong emotional pull). If you blur the edges between items of different types, your system will not be effective.
The practice of regularly and completely emptying all of your inboxes so that your trusted system is updated with the outcome of processing all of the new stuff. The opposite is trying to use, for example, your email inbox as a management system in itself.
The practice of setting aside dedicated time (typically a couple of hours) to make sure your system is clean, current and complete. David Allen regularly cites this as the critical success factor for GTD.