Is it your very most favorite thing to wear? Than wear it with the most important thing you will wear, a great big smile. This is a good time to push boundaries too. What's something you've always wanted to wear, but have been afraid to? The happier you are and the more you express yourself, the more others will enjoy it. Also, it's a great time to be creative. My playa wear is very cosplay inspired, but it is of an original character, me! Why not make a new cosplay from all your favorite aspects of existing costumes?
Don't forget, you need goggles and a dust mask, you can make that part of your costume! Boots are a good idea too. During the day, you are going to want to stay cool. For some people, that means wearing very little. For others, it means wearing light fabric. Light colors make a big difference out in the sun. It is a good idea to have a hat to block the sun. If you are fair skinned, be cautious of sunburn. At night, it's a little different. I like to return to camp for a water refill, some dinner, and to change clothes at sundown. Long sleeves and layers are good to keep warm. Make sure to light yourself up so others can see you. Playa dust dulls colors, why not light yourself up with some brilliant colored LEDS! In addition to not getting flattened by art cars and bikes in deep playa, you will always be able to see where you are going and what you are doing. If you make a pattern that is recognizable from far away, your friends will be able to spot you in the distance.
When I cosplay, I often make costumes that are elaborate, but difficult to move around in or uncomfortable. At Burning Man, you are going to want to be able to walk far, squeeze through crowds, and climb on things. Are you a dancer? I really like having costumes that accentuate dancing motions. Or maybe you are less hyper than I am. Maybe you want a nice warm faux fur jacket to keep you warm while you sit around the fire under the star lit sky with your friends.
One of my favorite things about Burning Man is making and seeing beautiful and functional things. So many things in the default world are beautiful or functional, but at Burning Man you get both.