Anytime I use a monster (or any kind of adversary) straight from the book, I photocopy that creature’s page before the game.
This preserves a bit of mystery by not letting my players see which monster books I’m using that day — or, for the cagey veterans, which sections I’ve bookmarked.
Particularly with well-known games like D&D, my players can usually guess what they’re about to face just by seeing what part of the book I flip to. (“That looks like the Bs, so those are probably behir tracks.“)
Photocopying monsters beforehand not only avoids this, it gives you handy piece of scrap paper on which to scratch off hit points and take other notes during the battle.
What tricks do you use to keep your players guessing about their opposition?