1. Make sure that somehow, some way, you can have some private space. A loft bed with a curtain underneath is an excellent, low-key idea that gives you privacy and somewhere that's all your own. When you share a room, you lose that completely so it's important to recreate it however you can.
2. Invest in a good pair of headphones. Sennheiser is a good brand, and you can get a nice pair for $20. It gives you the same audio performance as a decent set of speakers and lets you listen to music or play video games without disturbing your sibling.
3. Arrange separate trips to the store. Get bedding you like, and don't show your sibling or let him/her know what it is. Your sibling should do the same. Your choices might not look all that great together, but they won't clash with the walls -which is the fastest way to make a room look bad- and you'll like your choice. That's all that matters.
4. If the room allows for it, get a screen. Hopefully privacy matters more to one of you than the other, because normal bedrooms only have one door. Splitting the room in half with a sliding shoji screen can make a surprising difference. Your sibling is still going to always be two feet away from you, but a visual barrier helps. If you have your headphones on and are playing loud music, you'll hardly notice your sibling at all.
5. Spend as much time out of the house as possible. Sharing a room becomes very stressful over time, but if you turn your room in to a place you sleep and store your toothbrush, it's not so bad. Do your homework in school or at the library, go to a friend's house, skateboard outside until dinner. Anything to keep you out of your room. Who knows, you might end up making more friends.
   

Tips

• Helping repaint the room and lay down the carpet will probably tip your parents towards saying yes if they aren't sure about some of the steps.
• A portable game system such as the Sony PSP or the Nintendo DS can help resolve conflict if you want to play a game and your sibling wants to go to sleep, or vice versa. In this case, the PSP may be a better choice because it also gives you the freedom to surf the Web or watch a movie, but that's a personal choice that you and your wallet have to consider on your own.
• Get a job and start saving so you can move out when you're 18 and have your own space.
• Don't expect your friends with their own rooms to understand. They can be great supports when you want to leech off of them or blow off steam, but they don't really understand the inevitable stress, pressure, and conflict that arises when two boiling sacks of hormones are confined in a small space.

Warnings

• Hold your ground. While you do have to learn to get along with your family, most people have some place that's truly their own to express themselves in. You don't, and you need this. Don't let yourself get talked out of it.
• Screens intended for room division run on tracks. To keep the track clear, and to avoid breaking the screen, you'll have to keep your part of the room clean.
• Headphones and a neutral rug won't help you when you get locked out.