I am planning to have flat iron steaks for dinner tonight, but bought them without much of a plan. I expect that cooking them on a grill is best, right? Any suggestions in terms of marinating (necessary? suggested?) or saucing? Accompaniments?

You need to tenderize them, so a simple marinade of red wine vinegar, olive oil, sliced garlic and ground pepper should do nicely for a period of several hours. I don't know how thick they are, but grilling them over direct heat should yield excellent results.

Since you're home already this suggestion is not particularly helpful but the Korean beef short rib marinade galbi that you can buy at Asian grocery stores is really good on flat irons. We marinate for a 30 minutes up to a few hours and then throw them on the grill and cook rare. They are really great cuts of meat, very juicy and very beefy in flavor. You do need to be sure to cut against the grain for serving.

I don't agree with this suggestion. I don't think they need tenderizing. I'd just season with salt and pepper (perhaps rub with a piece of crushed garliic) and grill over charcoal or cook in butter in a cast iron skillet using medium heat. Flat irons, in my experience, are not as thick as other cuts so I would cook for 3 minutes/side and then check for doneness. I would serve with some roasted potatoes and corn on the cob.

Thanks all. Generally I can't use prepared marinades because my +1 is on a low sodium diet. For these steaks (which are about 3/4" thick), I'll prep mine with salt, pepper and garlic and his will be minus the salt. I'll let you know how it goes!

At 3/4 inch, I would do 4 min per side and then check. How will you be cooking then, on the stove or on the grill? Also, don't foreget to let them rest for at least 5 minutes after cooking, to let the juices redistribute.