What,if anything, should I season a very good quality skirt steak with before grilling?

I think that skirt, flap, flank, tri-tip are enormously flavored with a little help with wosrtchire, soy sauce, Dijon mustard, etc. - some type of marinade besides salt and pepper. This type of meat is fine on its own but it takes on so much flavor easily.

I agree that S & P is enough, but if you want a rub I like a mix of S & P, a little sugar, garlic, chili flakes, porcini powder, and a little EVOO.

Salt and pepper. Serve with lemon wedges. That's it. It's kind of a Tuscan thing. Hopefully you are grilling this "very good quality" steak over a real wood fire and not some wimpy gas apparatus.

I do like to let the flavor of the steak shine through when it is a good cut but I also find that a little seasoning helps. In addition to kosher salt and black pepper, I very lightly dust steaks with garlic and onion powders. Then, let the steak sit at room temp for about an hour.

I only used good sea salt. When using a not so great quality piece of meat I would marinate it in a little soy sauce, brown sugar, rice vinegar and garlic. Good luck!

Salt!

I personally find that Excellent meat benefits from very little. Pepper, Salt, and Garlic are my staples. Occasionally I'll do a marinade or a rub of some sort - but I like the flavor of the beautiful beef to be center stage.