I once worked in an office where I was increasingly assigned task after task. First my direct supervisor retired, and I was assigned 75% of her work load. Another employee quit and I was assigned 50% of her workload. So, I was doing the work of 3 people, training and supervising my department in the increasing absence of my manager and new direct supervisor (when they were in meetings with upper management). A third-party company was hired to help our department and I was tasked to train them too. And then, I had to clean up their messes. It got to the point that working through lunches and breaks wasn't enough, and management expected overtime during weekends. That was a red flag and no-go for me as I had a one-way 1 hour 15 minute commute. I knew going in on the weekend was detrimental to my mental health, which wasn't very good as it stood.
Set Boundary #1 (no weekend work) - I refused weekend work, except for ONE time in which I was given a laptop to work from home...during a major holiday (RE July 4th). I told them "I had made plans", which I did not explain other than they were personal and I could not cancel. No one had any right to know what I had planned outside of work. No one has the right to know what you plan outside of work. It didn't matter that all I was doing was unwinding after the week. I deserved to unwind after the week! I honestly think they were shocked that I didn't care about the money, and I was "unbribe-able". My primary concern was quality, and they were concerned with quantity.
Set Boundary # 2 (refusing work/new projects) - I realised that I was letting them abuse my expertise and experience and kindness. I could do quick and accurate work and they knew it. I started asking management which task they felt was most important when they wanted to assign me a different new task. I laid the responsibility of choice on THEIR lap. I would say something to the effect of: "Okay, I'm doing (tasks A, B, C, D, E and F right now and provided some deadlines), and you want me to do tasks X, Y, Z. Which task do you want me to complete first and which is of highest priority to you?" It only took doing that three times for them to get the picture that I could NOT take on additional work. This was me finally getting to my breaking point and being the extremely passive-aggressive dragon that I am on the inside that I don't let show. I am always professional, quiet and patient to everyone. On the inside I'm an anxious wreck. Something had to give. It got my point across effectively. I never actually said "no" to any new project! And, that fact makes me chuckle to this day.
I don't have that job anymore. My department was outsourced, so I was laid off.
I learned a valuable work skill: the diplomatic way to say "no", without actually denying to do work.