In an ideal world, you'd arrive at the office, sit down in front of a list called "things to accomplish today," and calmly work through each item until it was time to go home. But most of us can only dream of having that much control over the course of our day. In reality, the modern workday is a minefield of unexpected tasks, problems, and requests that blow up at completely unpredictable times, often, one right after the other. So instead of working our way through a prioritized task list in order, most of us get into a bad pattern: We just constantly react to and do whatever's in our face at the moment.
Of course, you've GOT to attend to certain interruptions. If the boss stops by to talk to you about something, you're not going to tell her to hold on while you wrap up the email you're writing. But when you always let interruptions determine what you're working on, you've fallen into the "busy trap."
Productivity expert David Allen says "the busy trap" is getting into the pattern of reactively handling the latest and loudest issues, instead of proactively deciding what's the most important thing to work on at the moment. The key to avoiding the busy trap is to make decisions, early and often, about what your priorities are today.
There are two modes of work: doing stuff, and deciding what stuff you have to do. The essence of working smart is balancing those two. If you do before you decide, you get stuck in the busy trap.
For office workers, "doing" work is writing email, filing paperwork, making copies, picking up the phone and talking to the right people, going to meetings—the physical actions are easy. But you don't get paid to do: you get paid to DECIDE WHAT to do. Defining your work is even more important than doing your work. It's only when you decide that you most effectively use your time.
You want to think first, act next. Because you never know when the next interruption is coming, make sure you always have a broad overview of all of your projects in one place. Regroup and review your lists on a regular basis—every day in fact, and on a larger scale, once a week. When you have clarity about what you need to be working on at the moment, when the boss stops by and asks if you can be on the committee—and oh, by the way, the meeting's in 15 minutes—you can say "sure, no problem" or "I'd love to, but can I delegate project X to make time for it?" instead.
The fact is, interruptions will always come at you in the form of emergencies, requests, surprise meetings, co-worker problems. That won't change. What can change is how you react to them. When you're clear on the state of your current projects and priorities, you can make informed decisions about how to deal with that interruption, the right way, on the spot.