Subaru is known for having HORRIBLE paint on their cars. To prevent the rock dings, don't drive behind trucks, lol, but there isn't much you can do. You could buy a bra for the car and that would help, but it makes the car look horrible. Also make sure to wash and wax the car, because that helps with the bigger bugs.
As for the paint...make sure that you wash the mirror WELL. You don't want to be applying paint to a dirty surface. Then apply as little of the paint as possible (it depends on the damage but you can always add paint not detract paint). The paint is also self-leveling so it should even out by itself. In addition, if it's a really small area then you can apply it with a cotton cloth or paper towel so you blend it in. Also apply more than coat, and apply a clear coat when you're done. Then wax it in a week's time so you protect it.