Salt (it's on steak, but it applies to chicken and other meats):

Truth of the matter is that you should salt your meat about 40 minutes before it hits the grill. When the salt first hits a steak, it sits on the surface. Through the process of osmosis, it'll slowly draw liquid out of the mat, which you'll see pool up in little droplets. As those droplets grow, the salt will dissolve in the meat juice, forming a concentrated brine. At this stage in the game—about 25 to 30 minutes in—your steak is in the absolute worst shape possible for grilling. That moisture will evaporate right off, leaving you with a tough, stringy crust.

Give it a bit more time, and eventually that brine will begin to break down some of the muscle tissue in the meat, allowing the juices to be re-absorbed, and taking the salt right along with it.
What does this lead to? Meat that is both better seasoned and more tender and moist when you cook it.

EDIT: Do use kosher salt, not regular table salt. The larger grains of kosher salt (which should more accurately be called "koshering salt," as salt itself is always kosher—kosher salt is coarse salt used in the koshering process) are easier to sprinkle evenly with your fingers, and will also draw more initial moisture out of the meat to dissolve than table salt.
Note that the 40 minutes is a minimum. A day or two ahead is even better.
If you use coarse sea salt normally, use that. Kosher salt is cheaper, though, and very easy to apply.