I use a monthly trivia quiz that gets people of all ages involved.

http://www.funtrivia.com/

just make a board specific trivia group and let it rip. the quizzes run daily and the scores are taken care of by funtrivia..

at the end of the month, we post at the end of the thread, the winners and everyone cheers etc etc..

i find that when people get hooked on the trivia/day thing that it builds in other ways as to people coming out of their shells and helping in other forums..

2c