It is important to remember that when you cook tougher (but oh so tasty) cuts of meat (such as flank steak, skirt steak, chuck steak, and so on) you have basically 2 options:
1) Either long-cook/ braise the meat to a slowly-moist-cooked, fork-tender well-doneness that you can literally pull apart with a fork. (Slow-cookers, Dutch ovens, and Foil-covered in the oven are perfect methods for this.)
or
2) Marinate the meat first (for several hours) and quick-cook (being careful not to overcook) to no more than medium-rare-pink. This will keep the meat at its most tender and usually requires a bit of attention and timing. Start with your meat at room temperature (pull from the fridge about 30 minutes before cooking). At end of cooking, pull the meat from your heat source just a little under-cooked (remember meat tends to cook about another 5 degrees or so, while it is resting!). Let the meat rest for 10 to 15 minutes (to set its juices) then cut the meat into thin slices across the grain. (Grilling, broiling, and even pan-frying are perfect techniques for this.)