Requirements: hard cover, plain solid cover, lays flat when opened, has some sort of closure.
Reasons: I like sketching in graphite but hate smudges. A hard cover keeps the sketchbook from flexing which reduces pencil smudges. Also a closure reduces smugging by keeping the pages from sliding side to side. I like a plain cover so I can embellish a bit and laying flat is a must so I can easily draw on the whole page.
Moleskine Classic Sketchbooks (Pocket) now and I'm hooked. I love the little pocket in the back, how flat they lay and how thick the pages are too help things from bleeding (although Sharpies still very much bleed).
Requirements: fits in pocket, lays flat, hard cover, has some sort of closure. Bonus has a pocket.
Reasons: Fitting in the pocket is a must because I don't carry around a purse. I love the flatness of this sketchbook because I can draw from one page to the next which allows for larger sketches. The hard cover and closure helps to reduce pencil smudging just like with my larger sketchbooks. I love the little pocket in the back which allows me to stuff little treasures away.
Erasers. I have a skinny Paper Mate Eraser Stick for precision and a Staedtler Mars Plastic Eraser cut in half for bigger jobs. I try to avoid using the erasers on the mechanical pencils because they are a pain to change.
Pencil Case. I've had a Caran d'Ache Metal Tin case for years and I love it. I plan to order two more so I can have one for pencils, pens, and lettering.