Instructions
Determine where your costume needs some livening up. To make an exciting costume, no area should be left without detail. Look on your costume and check to see where there is an extended length of undefined fabric, or simply exposed skin that looks like it could use some extra flare. If you're ever at a loss, ask a friend if there is any part of your costume that seems boring.
Add accessories that complement your costume. If the waistline of your costume is empty of detail for example, it would be appropriate to add a belt or maybe a cell phone hanging from a chord or string. Bare arms or hands could use gloves or bracelets. Jewelry can be helpful for just about any area of your body. Hair can use any number of hair accessories or a hat.
Use makeup and temporary tattoos wherever accessories don't do the trick. You can accessorize as much as you want, but there will usually always be bare skin that can use detail. Note that scars and tattoos can tell a lot about a character, so feel free to get creative with body paint and makeup in order to add that extra touch to your costume.