We were working with the Washington Wild Things (a minor league baseball team near Pittsburgh) this summer on growing their text messaging list during games. They were running a promotion during the first month of the season where fans at the homes games could text-in for the chance to win a prize given away at the end of the month. Because the prize was of significant value we felt the number of fans texting-in each game could be higher.
During the second month of the season we encouraged them to hold the text-to-win contest completely during the game. Fans were asked in the second inning to text in for the chance to win a prize or to play an on-field game during the seventh inning. Relative to the prize given away before these prizes were somewhat smaller. However, the opt-rates per game were much higher. Fans were more eager to enter the contest when they knew the prize was going to be given away in just a couple innings instead of a couple weeks.
As with any SMS marketing campaign the incentive for joining is always crucial to building the mobile marketing subscriber list. Making the incentive more immediate will increase the opt-in rate, even when that incentive is smaller.