Institute a punishment for the loser
This one is fairly simple. Punishments for the loser have been a fantasy league staple since fantasy sports started. The punishments could be classic embarrassment like holding a sign on the highway saying they lost. Or you could throw a classic lacrosse twist on it: loser steps in net? Loser runs the gauntlet? Either way, while you need a prize for the winner, its perhaps even more important to have a punishment for the loser.