Then, I start initiative and resolve everything one PC action at a time. This suits my style of GMing because I can tackle things in a realistic way on a micro scale. I just have monsters and NPCs react or counter-act accordingly to each round's actions. The big picture takes care of itself. I also find that this method encourages better PC planning and teamwork.
Plans tend to go awry when it's chunked down to individual and individually-controlled actions.
And all the PCs have to clearly understand their role in order for the plan to work. However, if it suits your story, pacing, game play, etc. to deal with a party plan on a bigger scale then I still do that. But, working with individual actions, rather than group plans, can often help you multi-task by breaking things into smaller, more manageable pieces. Let them go on for a couple of minutes before bringing them back on track. When players are planning things, let them plan at liesure until you're all caught up, and then give them a deadline to reach a decision to get things moving again.