One of the best decisions I have ever made as an artist is to buy a sketchbook small enough to fit in my purse, and start carrying it with me. Whenever I find myself waiting somewhere with nothing to do, I sketch whatever I can see.
This same advise can be found in many books, but I had to read it a number of times before I finally purchased a small sketchbook of my own. You should shop around to find a sketchbook that suits your needs. My choice is a Handbook, sized 5 1/2" x 8 1/4". It has a strap that keeps the cover closed, to which I can attach a mechanical pencil and a couple of pens. It also has a nice weight of paper, with a little tooth to it. It is perfect for pencil or ink sketches, and also handles light watercolor washes just fine. You can probably purchase a similar one at your favorite art retailer. I got mine from Cheap Joe's Art Stuff. (The best place I know of! CheapJoes.com)
What do I draw in my sketchbook? The answer is, anything! I draw the objects on the table at restaurants, or the view outside the window, perhaps. However, my favorite thing to sketch is the people around me. Sketching people in my sketchbook has improved my portraiture skills dramatically. Drawing in my sketchbook is different than drawing anywhere else. It's just for me. There's no pressure to "not mess it up", because I don't have to ever show it to anyone if I don't want to. Also, the people I'm drawing usually don't know that I'm drawing them, and may move at any time! This causes me to draw more quickly. I don't waste a ton of time thinking about it; I just do it, and therefore get a lot more practice because I'm not spending so long on one little sketch, just trying not to "screw it up"!
Whatever you draw in your personal sketchbook, the most important thing is to remember that it's just for you. Use it to your advantage to practice your drawing skills, and pass the time, too!
A recent drawing from my sketchbook, my grandfather, Frank, while we were waiting at the doctor's office for his appointment.