What is a good marinade for steak?

If you are using a good quality steak like T-bone or Rump I personally don't marinade them at all. I usually rub them with olive oil (don't oil the grill oil the meat) and then season with salt and pepper ( I use a grinder). Make sure your grill is hot and then gentle place your steak on to seal. Only turn your steak once. As a rule of thumb rare is 3 min/side to well done about 8 min/side. A method I like to use to check the "doneness" of steak is using my thumb and fingers. Place your thumb on the index finger of teh same hand, and feel the fleshy bit at the base of your thumb, this is rare, compare to the steak. Second finger is medium rare, third finger medium and fourth finger is well done.