Tell them what you like about it, how you are benefiting from them, etc.
Nothing works better then a personal recommendation from a trusted individual.
Try a personal testimonial to boost your commission checks instead of the old banner advert. Don't get me wrong banners work well, but your own testimonial works even better. Use a combination of banners and personal testimonials throughout your web site, newsletter and email. You will be surprised at the increased interest. Try it for a month and document the changes in commissions and new sign up's.