I kinda use an "aggro" thing: You attack a monster, it generally gets a chance to attack back. That's not to say that I don't allow for focus-fire to drop a monsters before it gets a turn (I feel that I'm pretty generous about that - partly because it means that I can save the time by not having to roll a turn for that creature!) but generally, I "activate" my monsters when the PCs interact with them, and do everyone else at the end. (Usually, I make NPCs act last).
It can get a little weird sometimes for people trying to follow along, especially when I do things like resolve all movement before I resolve all actions (something I tend to prefer in my head, because it keeps the action flowing closer to simultaneously). Also, I play fast-and-loose with timing when I write my descriptions (or in other words, my "fluff" doesn't always match exactly what happened mechanically, at least not when it comes to order-of-events). But I do that just to make it easier to write, not to mess with the game.
It's not perfect, but whatcanyado?