One of my coworkers is great at pushing back with some humor: “I’ve currently got X, Y, Z consuming all my time, which should I shift for A?”

Personally, I’m not confident enough in my efficiency to reject work (I always think I’m slacking or too slow). I just send up dim warning flares via email. Vague ones: “I’m trying my best with my workload, but I think things may start falling through the cracks…” Specific ones: “X may contact you about the delay in getting Y. I haven’t had time to study it yet.”

Then, if they do, at least I gave some warning.