I mentioned before that I like my tools to be as simple as they can possible be. Another example of this is my notebook.
I love Field Notes notebooks. They are simple, lightweight and provide exactly what I need.
Here’s one of my Field Notes, together with my Embassy Pen

Now, you might think that being a geek and a computer security guy I would not use a paper notebook. That was true a few years ago, however I’ve found it works better for me when I write down my ideas, notes, tips and other snippets of information on an actual sheet of paper.
Writing it on paper allows for a much better way of transferring your thoughts into your notes. You can sketch things on the go, you can add side notations, little marks, etc. For example, this is an example of a sketch I draw after a recon I performed for a physical penetration test.
There is plenty of good software out there that can help you with note taking (like Notational Velocity and Simplenote), however I still prefer the feeling and the power of having written notes; besides, all personal notes should be handwritten.