Decide if you want to stick with the color theme of the uniform. Most uniforms have a two-color fashion theme, usually basics like blue and red or black and yellow, for example. When the two of you are choosing accessories, decide if you want to stick with those colors or add some contrast with other hues. For example, a blue and yellow uniform would really "pop" with some red ribbons in the hair and red jelly bracelets. This is the kind of thing that makes your child stand out, but not in a way that distracts other kids.
Focus on funky hairstyles. Adding bows and ribbons to your child's hair is a great way to dress up a school uniform. Barrettes come in all different shapes and sizes. Put a few in your child's hair and invent new hair styles together. Spend as much time on their hair as you would on yours if you were going to a special event. Let them experiment with styles that are both up and down and don't be afraid to trying something new or let them do something with their own hair. You could also add a headband or a knit hat into the mix for something more eclectic.
Add some color, stickers and/or glitter to their shoes. Your child's shoes can make all of the difference when it comes to adding pizzazz to a school uniform. Purchase a fabric decorating kit and a plain pair of white tennis shoes. Let them go to town, drawing on their shoes and adding glitter and/or stickers. It may look like a scattered mess to you, but to them---and their friends---it helps them to stand out.
Dress up a uniform with baubles. In any city, you can find a store that sells cheap, colorful, costume jewelry. Take a trip to just such a store and let your child pick out some jewelry to wear to school. It shouldn't be anything too large or anything that may cause too much of a distraction to other students. You should also refrain from purchasing anything that hangs (think dangly earring) or poses any kind of choking hazard. Items like jelly bracelets and teddy bear stud earrings are simple ways to dress up a school uniform with defeating its main purpose.