Accessorizing is one of the best ways to personalize your outfit. Even the "most boring" LBDs, which are always so stylish, can be made fun with nice piece of jewelry.
pic: net-a-porter; ONE VINTAGE, Dicha Tallulah earrings
For a cocktail party, I would wear with a LBD for example these wonderful earrings. They would not need any other accessories with them. Simple is always better.
Or optionally, the beautiful necklace (below).
pic: Net-a-porter, MAJE necklace