I'm reading a fascinating book (below), which looks into the sketchbooks of various artists, who talk about how and why they keep them, what their particular hang-ups are, and any personal 'rules' they impose.
Pretty much everyone agrees it's 'against the rules' to tear a page out, no matter how appalling the sketch. Another common rule is that books must be filled to the last page. Personally, once I get near the end, I worry I'll run out of pages over the day, so take a fresh one out instead. I always start a new book for any new trip too, often finishing only part way through, so I generally have 3 or 4 half-filled ones on the go, which I add to randomly.
Another factor several people mention, is the stifling effect of success: the better you get, the more obsessed you can become with the need for every new drawing to be a thing of great beauty. If you are not careful, this can be crippling.
In recent months, since I've been posting my sketches here and on Urban Sketchers, I have noticed myself starting down that route. Not good!! So, in my new spirit of experimentation, and in an attempt to return to sketches focused less on results, I have set aside my pencil for a while.
Instead, I have bought myself a pen. Ink is a medium I've never used for sketching. This means I am allowed fairly low expectations for a while, which is very liberating. I tried it out over the weekend. The results were rather washed out, so I tinted them with watercolour and waxy pencils. The ink wasn't waterproof, so I sometimes got in a bit of a mess, but had fun playing around, bringing the drawings back from the dead, and was noticeable less uptight about whether it ultimately worked or not.
I have bought some waterproof ink too, so I'll see if that's less messy!