Sharpie for me too.

So far as writing on a person's skin where the sharpie doesn't show up so well, tear off a piece of tape (white, blue etc), write on that, and put it on your patient.

For mass casualties I have drilled to put a 1 ft of tape on my pants leg, and write important stuff - search results etc - on that. When you go in to debrief, tear it off and put it on the back of the search results. Same for doing triage - if you don't have patient tags or paper, write down vitals, bp etc, and transfer the tape to the patient or their medical record. Mostly works, as long as you're diligent about marking your written stuff according to patient (get a name or patient number). If I'm really thinking about it I would probably do it for a single casualty but haven't had that kind of urgent care where it was necessary.