What's a good sound-system set-up for open areas?

I have been with our school orchestra for a while now. We always play in closed areas during our events, and usually, we use 2-3 condenser mics (for the wind instruments), 3 monitor speakers, and sometimes a percussion microphone. This time, we are playing in an open field, so probably, the regular sound equipment we use are not going to be that efficient. What should be a good sound-system set-up for an open area for a 30-man orchestra?