“He who trims himself to suit everyone will soon whittle himself away.” Raymond Hull When you block out time for yourself, make sure you know what you want to get done during that time. When you have blocked time that is not specific, then it is easy to give it away. You want to be able to see what you have to get done so that it weighs on you and you see it as important to accomplish. Then when someone else asks you for something or wants to take your time from you, you have a frame of reference to make an informed decision as to giving up your time or keeping it.
You can also suggest an alternative time to help them. I suggest to clients that they create “unofficial office hours.” This is time in a day that is unscheduled that you can suggest to people as a good time that you will get back to them to help them or they can get back to you.