Are you a spine breaker? Or a dog-earer? Do you expect to keep your books in pristine condition even after you have read them? Does watching other readers bend the cover all the way round make you flinch or squeal in pain?
I usually use bookmarks. I am not above dog-earing (actually Russell caught me at this the other day and was horrified), but I really only do it to certain types of books: books I receive from my dad who is a dog-earer, BookCrossing books that are already pretty beat-up. If I'm reading a clean/new/nice copy of a book and I don't have a bookmark handy, I just try to remember what page I was on (or what chapter or section I was beginning).
I'm very careful with library books (nothing annoys me more than getting a book from the library and seeing it marked up with someone's underlining and notes) and books loaned to me by others. I think you should treat other people's books with respect.
I remove the dust jackets before read hardcovers (unless the book in question is a library book with the dust jacket firmly attached). I find that dust jackets get in the way when I'm reading and they just end up getting banged up.
Books I buy for classes get marked up as necessary while I am reading, underlining, some marginalia. For me they are working copies and I treat them as such.
I'm more disrespectful with mass markets. Sometimes you just have to break the spine in other to read them (and, honestly, they aren't made to last). I hate reading borrowed copies of mass markets for just this reason. I'm fine if the owner has already broken the spine, otherwise I go crazy trying to read while keeping the book in good condition.