In my sketchbooks I allow myself some room for experimentation, I always go back to watercolor anyway. A sketchbook is vital because it is where I push my ideas and build my skills. I like to practice my skill to find improvement and to experience the freedom to make mistakes. This often leads to a new way of approaching an old idea or technique.
I carry a small bag for my sketchbook and pen. Repeatedly I will find myself waiting somewhere with a few minutes to devote to sketching. There are many ways to approach sketching. I love the idea of keeping it as a journal of events or memories by using a theme, such as dinner. Observing nature or architecture, the possibilities are endless. Decisively, it is very important to draw from direct observation. I have been taught to look more at the subject while drawing, than at your paper, this much has stayed with me.