Do you find yourself saying yes to every request that you get from your boss or your colleagues?
Maybe you are trying to be nice to them, but remember that when you do so, you are putting aside the things that you had in your schedule, so please learn to say no.
First, you need to be clear about why you are saying "no." This will help you stand by your decision and avoid guilt. If the request is unreasonable, but you still want to say "yes," perhaps you can offer a reasonable alternative.
Try to commit only to those tasks that support your goals. While you may be tempted to commit to a task outside of your stated goals to be a team player, you should only do so if it does not jeopardize your other work. If you do not have the time required to devote to it, you are not helping anybody by committing to it.
Learn to say "no" to non-priority projects, even when they are presented by management. List the projects you are currently working on and ask the requester to decide on priorities when he or she adds new projects to your responsibilities.