I found keeping a timesheet helped me there. When it came to "it is getting to the point where X is not going to be ready on time, and I've got personal things on this week so I really can't work significant overtime", and the question came back "well, what have you been doing for the last week", I could give them a full list of "2 hours helping X with Y", "an hour doing Z", "four hours on that high priority fix client C that no one else was willing to touch" and so forth. Don't play the blame game and point out that person X was being thick on an occasion or otherwise dwell on who is interrupting your key work, just lay it out as matter-of-fact as you can: "This is where my time is being used".
Make damn sure though that people understand a short interruption "for a quick meeting" when you are at the time "powering through" stuff sometimes kills as much as a full hour of development time if you are working on something complex as it can completely break your train of thought (especially if you are tired due to recent overtime!), or worse if you are coordinating with other people and/or are working on something timing sensitive (so the interruption might mean a key coordination point is delayed).
If you can show what the demands on your time are, a good manager will try rearrange things so that you are interrupted less often. Sometimes simply merging those small interruptions into a scheduled period or two each day is enough to make all the difference to your mid/long term work: problems that didn't really need you magically go away (as the person who was going to interrupt you does that 30 seconds of extra thinking/research that was needed to give them the clue) and the others "interrupt" you a few times a day rather than several times over an hours or two.
Make damn sure though that people understand a short interruption "for a quick meeting" when you are at the time "powering through" stuff sometimes kills as much as a full hour of development time if you are working on something complex as it can completely break your train of thought (especially if you are tired due to recent overtime!), or worse if you are coordinating with other people and/or are working on something timing sensitive (so the interruption might mean a key coordination point is delayed).
If you can show what the demands on your time are, a good manager will try rearrange things so that you are interrupted less often. Sometimes simply merging those small interruptions into a scheduled period or two each day is enough to make all the difference to your mid/long term work: problems that didn't really need you magically go away (as the person who was going to interrupt you does that 30 seconds of extra thinking/research that was needed to give them the clue) and the others "interrupt" you a few times a day rather than several times over an hours or two.