I marinade just about everything, but not steak. Marinating a good, aged steak is sacrilege.

A little bit of olive oil to retain moisture, salt, pepper, and maybe some garlic and thyme. That's about it for me. If it's not the best steak, or I'm making some kabbobs or something, I'll go with a dry rub with the above ingredients and perhaps some paprika. I also take a sliced onion add in some diced red pepper and place it in a foil pouch with spices and olive oil. I grill that package along side the meat and place that mixture on the finished steak. Mmmm, Mmmm, good!