If it's a fun theme that both the students and the mentors can really get into, they'll want to stick with it year after year and build fun traditions. Our team (which has a Monty Python and the Holy Grail theme, as a play on our school's mascot, the Knights) presents a marketing/branding workshop each year with a lot of good tips. The power point from the workshop can be found on our website, under resources:
http://firstteam1939.com/wp-content/...iSpirit121.pdf
Good luck!