As for marinades, I had a post in here somewhere about "brining" pork chops or chicken. You gotta try it - you won't believe the difference.
Sorry, no marinade ideas... A butcher shop near Chicago has a marinated skirt steak that's so tender that you almost don't need a knife to cut it. Regular skirt steak is very tough - but has great flavor. I haven't figured out what they do to it, and neither has my friend the chef... Ziploc bags are great for marinating or brining. Put the meat and marinade in, press the air out and seal. Every so often, mix it up a little (kinda like agitating film ).