Sequins
Sewing your own clothes is great fun, but there's a difference between clothes and costume, and for a costume you're going to need a little bit extra. Enter the humble sequin; a simple, shiny embellishment, usually made of plastic, that is easily sewn onto fabric through a hole in the middle of the sequin. Sequins are almost universally shiny and reflective, and come in all sorts of shapes, sizes and colour gradients. From simple circles to flowers and snowflakes, sequins are a supremely versatile tool in the sewers toolkit. And while a ton of sequins can really finish off a stage costume, there's no rule against adding a tasteful amount to a top meant for everyday wear. One of the keys to making sequins look good is to pick at most two types of sequin and then use those throughout. Sequins can very easily make your project look busy and ugly if you go overboard.
Trims and Tassels
Besides sequins, there are a few other things you can easily apply to your next sewing project to make it look just a little bit nicer. You can put on anything from patches to rhinestones, but sewing trims and sewing tassels are a great way to continue the aesthetic you've established with your sequins without making the surface of the garment too busy. Rhinestones can be used in much the same way as sequins, but you can use larger ones as well to create a design to break up large areas that would otherwise be monotonous. Other appliques include iron-on patches and lace bordering, and other beads of all kinds.
Buttons and Zips
Some parts of a sewing project are a practical necessity; such as buttons, zips, and other basic parts of a garment's functioning. That doesn't mean they can't also be part of the design and become an embellishment, however. Matching or contrasting the colour and style of the buttons on a project to the sequins or beads or other bells and whistles is a great way to create interest.