You're in the middle of an important task, and something pops up. Maybe a colleague or your manager comes over asking for or about something. Maybe an email or phone call interrupts you.
Do not stop what your'e doing to handle the new request, unless it's an emergency. Instead, keep a journal or use computer post-its on your desk top, and note the task down. After you've finished your current task, you can take a look at the new request and figure out when to slot it in your work schedule.
This will keep those boundaries that are so important, as well as a great work/life balance.