Trying out for captain can be a little intimidating when it involves multiple steps like making a long routine.
The first thing to do is relax, I know it can be difficult when you really want the position, but if your instructors see that your are calm and confident, they will be more impressed with that than with performing a flawless rendition of the flag routine of the century.
The next step in choosing music. Think about your personality, are you up beat? Then maybe a hip-hop song like 1-2 step by Ciara is for you. Are you the wild child of your team? Maybe a song by Pink would work. Are you someone who likes to perform to really emotional songs? Try out Lifehouse's song you and me. A site with a pretty good list of songs for routines is
colorguardcorner.tripod.com . . ..
When making your routine, always play up your strenghts. So, if your flexible, you could make the ending pose of your routine the splits. Are you a talented dancer? Try puting some dance work under your flag choreography, like doing a chaine turn while passing the flag around your torso. Even if your only strong flag skill is a dropspin, try and spice it up by doing a chasse at the same time. Also, try to make up your own toss or dance move, name it and tell your juding panel about it before your routine starts. They will be impressed that you would go all out like that to show them your skills. You can also take basic skills like carves (also called stiring the pot, snake, and figure 8) and do it in reverse by making the cone shape on the bottom first and then the top cone shape.
Lastly, make sure you know how to perform your routine. Keep your chin up above the horozin and never make a facial expresion that acknowledges a mistake, remeber this is your own unique routine, they have no idea what is right or wrong. Also, see what kind of emotions are expersed in your music and portray them to your audience, so if your song is happy, smile. Doing a sad song? Don't make a pouting face, it just looks silly, instead, try and communicate the emotion with your eyes and the way you move. Going for a crazy song? Pretend you are a rock star and just let loose.
One more thing. Pull your instructor aside at the end of practice and ask if you can wear a costume and do your hair and make up for the try-out. If it is not already required, and s/he says yes, don't tell the other candidates for captain of your plans! It will give you an extra one up on the group and it will show your judges that you are really thinking about this and are dedicated to be in a leadership role. Hope I helped.