This is where maintaining a list of current projects comes in handy. Keep a version of it in priority order, with (if applicable) deadlines noted on it. When your superior asks you to add another task, provide them a copy of the list and ask them where on the list the new task falls.
In some cases, just showing them the list solves the problem, because they can see that you already have too much to do, or at least that the new task is so much less important that your current tasks that it will just simply never get to the top of the list. They may also take other things off your list to make room for it. It's important to remember that supervisors aren't psychic or perfect record keepers. They really may just not know how much you have on your plate. (I've shocked supervisors by giving them a written record of everything they've asked me to do in the previous week. When people are busy, they just lose track.)
If the new task still goes on your list, consider any deadlined tasks it's pushing down and let your supervisor know if the new task endangers you being able to meet those deadlines while working a reasonable number of hours. In all earnestness, do push to work only 40-ish hours a week. It's been known empirically since Henry Ford was building the Model T that worker productivity drops with excessive work hours. And, again, unless you're showing them a time sheet or something else explicit, don't assume your supervisor knows how many extra hours you're working.
Of course, being able to give those types of warnings depends are being able to judge accurately how much time each task will take you. For most knowledge work, that's a matter of record-keeping and practice. Keep a simple log of assigned projects, estimated time, actual time, and notes on any particular issue you forgot to account for or that made the task an outlier. When you get a new task, review the log for similar ones and use their results to help guide your new estimate.
Even when not negotiating where on the list new tasks go, be sure to show the list to your supervisor regularly. (Where I work, it's SOP is have a standing weekly half-hour meeting with one's supervisor, specifically for discussing things like project load, upcoming vacations, career development, etc.) Priorities often shift with the whims of upper management and keeping your list up-to-date with those shifts will help prevent unpleasant surprises later on. It also helps your supervisor have a better idea of how much is on all of their reports' plates, for when their supervisor asks if their team can take on more work. (After all, everything you go through with your supervisor they probably also go through with theirs.)
