The following five scans are excerpts from a sketchbook I carry with me during my commute on the Bloor line of the TTC from Danforth to Christie and other destinations during the week.
Sometimes I prepare my sketchbook pages with flat washes or textured backgrounds so my sketches have better depth prior to sketching on the pages. I find early mornings are good (people are sleeping and easier to draw or observe).
As a rule I try to sketch with a permanent pen, (no erasing). This keeps the work loose and fresh and adds a little edge to the drawings. It makes you have to concentrate more because a bump or wrong stroke can put a quick end to something you like.