Getting Things Done
Lists for every occasion. One list for current actions, a waiting list, a list for each of the people you have business with, and more. The use of folders to both keep reference material organized, but also to 'mail' yourself things into the future. A calendar and regular reviews of all your lists to keep your mind free from having to remember what you need to do. Last, but not least, how to put EVERYTHING into your inbox and still make sure that it stays relatively empty.