Give members the opportunity to challenge their friends to a friendly competition. When the competition is over, give a prize to the winner. The prize can be free class passes of their choice, a free t-shirt, or a discount.
Offer your clients a special offer for joining a program in the Spring. This could be a free workout, diet plan, or just a discount for existing services. This is a time-limited offer and should begin right before or at the beginning of spring.