A lot of great shows start with just one idea. If you're embarking on your field show selection, you're probably already tossing around words and concepts that could be your starting off point. To help guide that process, check out some of these tips for selecting a theme for your show:
How much can you interpret it?
The last thing you'll want is to have your chosen theme limit you. It should be broad enough where you feel free to interpret it as loosely as you wish. Too specific of an idea could force you into expensive or time-consuming choices for details like costuming, visuals and choreography.
"Consider how an arrangement can evoke your theme."
How will music reflect this idea?
Since one of your next steps will be choosing the music to accompany your show, you'll need to think about how an arrangement can evoke your theme. When considering different ideas, write down words or moods that could be associated with it and think about how that would translate into music.
What will the choreography look like?
Next to your music, the visual representation of this theme is one of the most important aspects of your show. You'll want to dazzle your audience by communicating your ideas in an entertaining way. Certain themes will allow for pretty clear-cut visual interpretations, while others may take a little more creativity on your end. There is no objectively "right" decision between the two, so the choice depends on your resources.