For good thick steaks...
Marinade in...
1 Part Soy Sauce
3 Parts Pineapple Juice
Don't worry.... it doesn't make the meat sweet. The pineapple juice is the tenderizer.
Let it marinade at least 4 hours and overnight is good.
Let it marinade for a few days and your steaks will disappear.
Of course we then sprinkle Bolners Fiesta Fajita Seasoning on them right before grilling and VOILA... good stuff