How do you handle the tidal wave of incoming stuff that lands on your desk? How do you surf it, instead of getting crushed by it?
Rigorous application of the "4Ds" is the key.
When "stuff" comes at you -- mail, email, phone calls, text messages, a knock on the door from your coworker -- consider that you have four -- and only four -- courses of action.
You can Do it. If it takes two minutes or less, just take care of the task there and then.
You can Delegate it. If you're not the best person to handle something, or if you don't have the most current information, or if you just can't get to the task for three weeks, pass it to someone else. This may be a coworker, or it may be a supervisor. The point is that you need to move this item forward, on the next step towards completion.
You can Designate it. If it will take more than two minutes and you can't delegate it to someone else, then you have to designate (schedule) time in your calendar to handle it. "Later" is not a time. Will you do it tomorrow at 2pm? Next Thursday at 9am? Saturday morning? Be specific. You'll have to address the task at some point, so you might as well choose when you're going to do it.
Finally, you can Dump it. In a perfect world, of course, you'd do everything that people asked of you -- you'd participate in all volunteer activities, go to dinner with all your friends, attend lectures and the ballet, join the softball team, etc. But it's not a perfect world. So be realistic, make the decision there and then, and just say no.
If you apply the 4Ds rigorously, you'll gain greater control over the daily barrage of responsibilities you're asked to handle. You'll feel better, you'll sleep better, and you'll work better.