I like the advice about contacting Joe Newton. Well, that may be tough to do these days; but at least get his books. "The Long Green Line" is fantastic, though if you get his later books you can see that his training has changed. Also note that Joe himself talks about building up the program over a period of ~5 years--if your guys try to do what Joe's are doing now, you won't have a team left! Regardless, his specific motivation ideas are priceless.
Remember that, particularly in high school, "one year of growth equals two years of coaching." If you keep your kids out for the team, from one year to the next, you absolutely will see real progress. *Consistency* is much more important than individual killer workouts.
Group goals can also help, especially with the girls: maybe working to get five girls under time X on a particular course, seven under X+a, ten under X+b, etc. This way even the non-scorers know they're shooting for something important, and the early finishers will go nuts cheering their teammates on to break "that" time.
Finally, I enjoyed the suggestion about making workouts more game-like. Paarlauf is great for getting the kids to work together. Use your imagination with paarlauf: you can have boys be partners with girls, or have runners of different abilities/experience running different-length loops (and just coming back to a common "exchange" zone to touch off their partners).
I think the fact that you're interested enough to post your question shows that the kids will be lucky to have you. Good luck. Enjoy yourself! Learn lots!