Think big. Really big. If you want your team to get a boost from your banner, then they're going to have to be able to see it. So why not go for the longest banner you can, and then draft your friends to help you hold it? Try and get your hands on several bed sheets or other large pieces of cloth, which you can sew together.
Get some spray cans and express yourself. Graffiti is only frowned upon when it's performed illegally on others' property. You'll be able to create some outstanding effects and you should familiarize yourself with the variety of styles such as Wildstyle, Block and Oldschool and then you can pick the best style.
Make your banner double-sided. Instead of just one cheer banner, you can make two and have another message by turning it around. You'll be able to begin the game with an optimistic sentiment such as "Go crush them Tigers" and if your team is struggling at half time you can switch it around to "Never Give In... Never Give Up." You know your team better than anyone else, so pick the sentiments which will be most effective in good and bad situations.
Pick your paints wisely, as you'll need to be noticed. There are a wide variety of paints out there designed specifically for that. Go day-glo if the mood takes you, or bring some glitter paint to your creation if you think it will help. Puff paint can give your banner a cool 3D effect Go to your local arts store and look what type of paints are available and then consider if it will help make your banner better.