Why do my teriyaki steaks come out hard and dry?
Any number of things can go wrong.


Is your flesh cut correctly, that is, against the grain? Hint: if you see a string, it's cut with the grain and will affect tenderness.
Has your flesh properly aged? Enzymes in the meat dissolve the tissue, making it naturally tender. 21 Days is recommended.
You marinade too long. 15-30 minutes is recommended here for your cuts.
You cook the meat too long. A thin slice should be fried on high heat for a short time.
Restaurants get better meat. Fact of life.


Edit: Basic sauté method.


Heat the pan. The pan is hot when a drop of water becomes steam instantly.
Add some oil and heat it. It should become very hot very quick. Look for wrinkles on the surface.
Add the meat.
When you see a drop of 'blood' (that's plasma) flip the meat.
Retire the meat.


[Optional]


Add cold liquid to pan (could be the marinade), clean the bottom of the pan.
Lower the heat and make a pan sauce.
Add the meat to the sauce for a minute and serve.