Stands for "Make 'Em Take A Loss", which is the philosophy that anything done can be done better by using all materials provided to you by a commercial enterprise.
Examples include using all towels in a hotel, even though you don't need them, taking both hot AND mild sauce when at Taco Bell, even if you have no intention of using them, using the automated handi-door activation so that you don't expend energy opening doors and, the best one, leaving your hotel lights and TV on at all times so that the room is ready to go when you come back.
If it doesn't cost you more, you should METAL to balance out the equation. It costs them a little bit more, and gives you a better deal than the next guy.
Remember to METAL the bar by taking 4 packs of matches, even though you don't smoke!