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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.