In the last campaing that I DM'd, I used a six-second rule for combat. Before combat began, I would explain the situation, set up the minis, have everyone calculate their attack bonuses & prepare. Jot down the initiative order, then tell each player they have six seconds to act. Count down the seconds; if they haven't given a definitive action, they move to the end of the initiative order. Once you get through the rest of the actions, return to the player(s) who were delayed and offer them another six seconds. If they STILL don't have a clear action defined, they forfeit their action that round. You can decide if they return to their former initiative order or if they're now stuck in last place until they refocus.
Works very well, and makes the combat much more fast & deadly. Of course, you cut out the out-of-character chatter, as well. Six seconds is not enough time for player A to tell player B to flank monster X while player C prepares his bigbangboom spell while player D should... etc.