I imagine a lot of people think an artist's sketchbook is full of wonderful and carefully drawn pieces of art. Maybe that true for some, but mine is full of half-finished ideas and scribbles. Here are a few examples..
@witchita (the flowers on the left are similar to her arm tattoo)
@nikkidelmosh (2 minute gesture sketch – I kinda like it)
@aliese (this got messy due to the ballpoint pen)
Anyways, the moral of the story is that you should feel free to mess around and explore ideas, especially within the confines of your sketchbook. It's taken me a while to realize that but I'm starting to get the hang of it