Sketchbook is the best friend forever for everyone who creates anything at any field. The most successful artists and beginners at all times had their sketchbooks to record an idea or practice a skill. Unpolished, messy, random and bizzare - each sketchbook has a character and a story to tell, as if it's some kind of a living being.
I have mine since summer 2011. Since then it helped me to produce some really great ideas, as well as to keep track of my work through the most artistically challenging times. Paradoxically, most of my sketches never serve as a concept for a fully finished piece. Sketching is more like a record of a thought or feeling, which accumulates into the final idea. I use to draw in it, as well as stick some objects, leaves or newspaper lines, etc (just whatever I feel like), and also write notes, favorite lyrics or excerpts from books.
A sketch doesn't have to be perfect or clean or mean something whatsoever - sketchbook is about being free and experiment, most refreshing feeling ever. Which is why a sketchbook is probably the most precious thing the artist owns ♥