Here’s a post with some great tips and inspiration for keeping a sketchbook, from illustrator Chad Townsend:
How to Keep a Sketchbook.

From the article:

What to carry:

Nothing fancy! It doesn’t have to be a high dollar sketchbook. Remember, you’re going to wreck it anyway. What you DO need is something small and portable – you are going to carry it everywhere you go and it will need to fit in a bag or be easy to grab and go.

You only need: 1 pencil, 1 pen, a small sharpener and an eraser (I prefer a kneaded eraser for many reasons notably so it can mold into my pencil box).

Pencil Box: I spent years hunting for the perfect one that was metal and could easily slip into my front pocket (see photo). Fortunately, I managed to get a few of these as gifts from a company that no longer exists. Unfortunately, I have never seen any like them since. Note: If anyone knows where to get them, please hook me up! I would like to buy some to give as gifts.

Cover: In case you are wondering, this leather sketchbook cover is made by a company called Oberon. It’s a cover for those cheap hardback sketchbooks you find at Hobby Lobby, Michaels or Barnes & Noble like these Pro Art Sketch Book 5.5″ X 8″, 220 Pages, Hardbound, as it is sturdy and I can switch out sketchbooks when I’m finished with them. And they look cool on your bookshelf. I date and number mine. I bought this one about 15 years ago and it has traveled with me everywhere. My wife refers to it as my security blanket! This option helps overcome the mental hang-ups brought on by the investment in the book itself. If you are going to commit yourself to sketching, I highly recommend these Oberon guys – expensive but a good option. Note: I rub baseball glove oil on it to keep it in in shape.

Read more at How to Keep a Sketchbook.