I find that remembering that the person trying my patience is experiencing dukkha and caught up in the confusion themselves helps. I guess you could see this as cultivating metta but not in the abstract, but by applying a little "I wonder how things got to be this way for this person" to the situation. And then remembering all the times I was doing variations on whatever they are doing that is currently annoying me, and thinking about how I'd want to be treated. Also, a little, recognizing where losing patience leads. And finally (most important), asking myself what's arising in me that's causing the lack of patience. All while actually paying attention to whatever it is that's going on/being said! Seriously, this sounds like it would take a long time and take you away from the moment, but I find the reminders happen in the blink of an eye.