Be realistic. Interruptions at work are going to happen, so build a handful of minutes into your schedule each day for them. Never schedule your time so tightly that you can't handle the unexpected.
When interruptions occur, try to keep them as short as possible. You can always ask your coworkers or clients to send you an email or schedule a time for a phone call.
Prioritize your daily to-do list; after the interruption, train yourself to go back to where you left off.
Put a note on your door or on an outside wall of your cubicle that you're in the middle of a project and can't be interrupted.
Minimize your own distractions — turn off your email notifications; set up a timely outgoing voicemail notice and don't respond every time the phone rings; disconnect from the Internet if social media sites are too tempting; turn off the ringer on your phone so you don't hear text messages coming in.