The key to making your Christmas tree look fuller is to use a combination of ornaments, lights, and tinsel. Use a variety of different sizes and shapes of ornaments to add depth and interest. Hang the ornaments in clusters, rather than spacing them out evenly, to create a fuller look. Don't be afraid to mix and match different types of ornaments - try some shimmery and others matte, some shiny and others more matte. Make sure to add plenty of lights and tinsel to give the tree a glowing, festive look.