How to get cooked meat to be tender?

Occasionally a piece of meat, pork, beef or lamb, no matter how well cooked will be tough because it was bad quality to begin with, coming probably from an old animal well past its prime (I know what cuts of beef must be boiled, roasted or fried so that’s not a problem).

But if starting from good meat what are the tips and advice for cooking it tender? As I understand a bad cooking method can make even the best quality meat tough like leather.

slow cook it. you can do it in the oven without a slow cooker. i do london broil at 275 for 4 hours (after searing it on the stove) and i almost cover it with water. then i make a gravy out of the water after it is cooked. mmmm delicious and it falls apart.

also, after cooking meat, cut it against the grain. it is easier to chew.