It can be difficult to do. Sometimes it requires hurting feelings or stepping on toes. But it has to be done. Saying "no" is a timeless way to overcome self-management issues.
Sometimes taking on tasks or favors for others is no big deal; it meshes into our daily repertoire. But other times, tending to the tasks thrust upon us by others takes priority over our own work and our productivity is compromised. As professionals, we must learn to identify this narrow boundary, and be comfortable telling other "no."
Coupled with learning to say no, there are other solutions to time management problems. In a serious crunch time with a fast approaching deadline, going into what I like to call "ghost mode" can be helpful. Eliminate all distractions, inbound messages and all. This can be for as short as 10 minutes, or as long as an hour -- whatever it takes for you to peak in productivity.
As professionals, it is important to be confident in saying no. Although it takes some work, being able to say no maximizes productivity by allowing you to get the stuff that matters done.