Coat your grill or a grill pan with cooking spray and preheat over medium heat. While the grill is heating, combine the brown sugar, chili powder, cumin, salt, and pepper. Brush each salmon fillet with 1/2 teaspoon of the oil, then rub each fillet with about 1/2 tablespoon of the spice mixture.
Grill the salmon, flesh side down, until charred, 4 to 5 minutes. Flip the salmon and cook another 5 to 6 minutes for medium doneness. For well done fish, cook an additional 1 to 2 minutes. Remove to a platter and serve immediately.