Long necklaces can give you a casual look but on the dressier side. It would be great to accessorize simple clothing like a simple shirt or a turtle neck top with loud and long necklaces. If you want a more glamorous look, you simply need to choose an elegant chain long necklace with a classy pendant. For formal evenings, you can wear the classic pearl necklace long and nice.
A style tip, you can also play with your accessories and layer. You can pair up your long necklaces with other necklaces of different lengths. This would give you an instant trendy look. ;)