Flowergirl gives good advise about selection. And Ras I think we all share your experiences with not so tender cuts, though the rib eye and the filet mignon of beef should always be tender (and I'd march cold cooked stuff right back to the store for my $$ back if
these cuts are tough and hard to chew).
Now for marinating the not so tender cuts like skirt and round and some of the other alternative smaller pieces which can be made suitable for a saute broil or grill. You can brine with salt. This adds little flavour and allows the full taste of the beef to shine. I'm assuming you're cooking for yourself so we'll estimate a pound of meat. Use 1/2 C salt to 2 C water and pour everything over the meat. Let it sit overnight in the fridge. Prepare the following day.
Acids are a good marinade, but remember that lemon juice, vinegar and wine will begin a chemical reaction which is much like digestion so the meat may begin to look somewhat disintrigrated on top if left for 24 hrs. But, in my opinion, I like the acids and use a mixture of wine or vinegar or lemonjuice (1/2C), as well as a fruit juice like orange, or pineapple or peach (try to stay away from grape or heavily colored juices as these will affect appearance) 1/2C. I like sweet marinades so I add either molasses, or brown sugar, or maple syrup or plain sugar (1/4C down to a tsp per taste).
Now, for the seasonings herbs and spices. These add to the tenderizing effect if allowed to sit for a few hrs.
Use any combination of garlic powder (1/4 tsp), rosemary, black or red pepper (to taste), paprika, basil, thyme (a tsp of dried each and use as many different kinds as you chose. Use your imagination and think about your preferences to possibly add terriaki sauce, soy sauce, oyster sauce, some barbeque sauce...about 1/4C if you chose.
Now add enough water to assure the meat is covered. Place in fridge for at least 2 hrs and up to 5 hrs. Remember the longer the acids sit on the meat the more "digested" will the texture become.
It's hard to ruin a marinade Ras. Use your imagination and bon appitite!