Technique number one. Beat the meat. A meat tenderizer or a reversible meat tenderizer is the tool of choice. Most meat tenderizers come with teeth, but the reversible tool has a smooth service that I like if I'm trying to flatten meat more than completely tearing it up. The small teeth of the mallet will break down the fibers of the meat and make it more tender, the flat side is good for shaping the meat to an even thickness that helps it cook evenly on the grill.

If you are going to tenderize chicken or steak, it can be messy. Some people recommend covering the steak with plastic wrap, but I don't like this because I've seen the teeth of the meat tenderizer leave bits of wrap in the meat. However, the reversible meat tenderizer with the flat side will do OK with wrapped meats.  The other option that is pointed out in the comments is to use parchment or wax paper.

Technique number two to tenderize meat is brining your chicken or marinading your steak. This technique is particularly successful if the meat has been tenderized first with a mallet. Brining poultry and marinading beef share the principle that the process absorbs fluid and flavor into the meat. When it's cooked, the added moisture helps cook the meat from the inside out and it retains more moisture. It also helps to sear the meat at a high temperature to help keep the moisture inside the meat.

The third technique is slow cooking your meat or pressure cooking your meat. Pork ribs that are slow cooked will fall off the bone. Beef brisket in a slow cooker or even better in a pressure cooker will become tender as well. Heat and pressure will break down protiens in the meat fibers making the meat tender.

It's good to tenderize tough meats, but for premium meats, I'd only brine or marinade them. Beef and pork tenderloin is already very tender, the brining or marinading can be used for adding flavor to the meat, but I think it hurts the quality of the meat to pound premium meats with a tenderizer or to slow cook them.