I tend to use notebooks filling all the space, its a professional habit, as with Tim M my lab book must be written on every page, and my copper notebook must be space filling to prevent accusations of later alterations if it becomes evidence in court.
I usually start a new day's entry on a fresh page, but slash out any unused white space on a previous page. Every page is signed and dated, with important findings countersigned by my boss. Yes, things can get this anal in science.

In my other life, a police officers' "original notes" hold evidential value, wherever they are written. I must preserve the original copy, or it's 'destroying evidence'. So if I write a car reg on the back of my hand, I must photocopy my hand, sign it, date it, and have it countersigned and dated by a ranking officer.
