The key is to have Email Templates that are appealing to the eye and draw your customers in to seek more information and to not send too many out so that they get tired of the same e-mails over and over.
Here is an example of a time schedule you could follow to get interest in an upcoming season:
6 weeks out
Send general broadcast email to all customers. Tip: To avoid spamming, you could limit the email to the last 2 years worth of customers enrolled in a season instead of sending it to everyone.
4 weeks out
Send an email to the customers currently enrolled in the season going on. Make this email look slightly different than the one sent out a couple weeks prior.
3 weeks out
Send broadcast email to all customers telling them it is not too late to get their teams in.
1 week out
Send an email to all registered teams in the new season to get their deposits in to get 1st games scheduled.
Season start
Send an email when 1st games have been scheduled giving them directions on how to go to their online account and what to expect the first day.