Most foods, especially meats, need a bit of salt. Salt magnifies flavors and helps proteins retain moisture. When applied you will normally use much less than in processed foods.
So the first step is to salt the meat the night before. How much salt? Shoot for about 1/4 to 1/2 teaspoon per pound of meat (depending on your rub spices will depend the amount of salt to use - if your rub has salt in it lean to the 1/4 teaspoon) and apply it heavier on thick spots. Kosher Salt is the best.
You can apply the rub in advance or wait till you are ready to grill. The rub is mostly a surface treatment for flavor and bark. So you can apply the rub just before cooking if you wish. Moisture and oils will mix with the spices and herbs, heat will work its magic on them, and all will be amazing. Put down a thin layer of oil before the rub because many of the flavors in the rub are oil soluble. Spread the rub generously on beef brisket, not so thick on other, thinner cut meats.