What ever is handy usually, a piece of yarn, a piece of paper, a paper clip, etc, etc, etc. I haven't ever had a real bookmark that I can remember.
I lied, for "God's Debris" I'm using the receipt from Books-A-Million. For "Hitman", I'm dog-earing the pages.
Nothing, I just remember what page I was on. I even have a real bookmark (it's one that you put over both sides of the page, and it magnetizes together.. real fancy) but I never use it because I know once I do, I will lose it somewhere. lol And I can't stand it when I get a used/library book with bent pages so I don't even bother with that.
I usually remember the page # I'm on, or the chapter which I stop on. (I'll stop at each chapter if I have to stop within such a time limit and just call it) If I forget, I go to the chapter i last remember and flip through a few pages, and read a paragraph or two and if i can recall what happened and flip a page or two, and see if i was, and keep reading until it looks very familer and the pages ahead dont look familier at all.
When i start reading a book, I usually don't stop until I finish. The only time I close the book is when I have completed it or I have to do something that deserves my full attention (i.e. driving) in which case I either place a finger where I left off or keep the book open and complete what I have to have done first. I'm kind of nerdy like that.
I remember the days when I used to be able to do that, it was before I had kids. I used to be able to read a full length novel, (300 - 400 pages) in about an hour and a half.
Also do that sometimes when I absolutely have to do something. Like, when I'm cooking something and reading .. well I have to stop to make sure it doesn't burn and what not.