I feel we should make a list of things to do on that day and give priority numbers and following the list is one way to avoid such problems. If any new issue comes, add that issue to your list and continue with your work. If it is to be attended immediately, see whether you can delegate it to somebody else. If there is no way out, then only you stop work on hand and start the new issue. This may avoid unnecessary cluttering of issues in the mind.
Planning of our works is also very important. Attempt important and serious issues first and then go for the other issues. That will make your mind free and you can focus on the job you are doing.