By Applying a thin inner layer of frosting to your cake it will trap crumbs and will help prevent bits of cake from appearing on the finished surface of the frosting, particularly important with the contrasting colors of chocolate cake and white or cream cheese frosting.
Using an offset spatula is the best tool for applying the frosting. Around the edge of the bowl you will scrape off any excess frosting on the bottom of the spatula, along with oozed filling and cake crumbs. Clean frosting gets picked up from the other side of the bowl. This way you will not spread any picked up crumbs back into the thin frosting coating.