Once your programme has been running for a while you'll need to do a few things to make sure they are happy and continuing to grow.
Site Changes:
If you decide to make changes to your site then make sure you inform the affiliates, please ensure you inform affiliates in advance of making such changes giving them time to update their own sites. If you change the product pages and do not let your affiliates know about it, then you'll find affiliates linking to pages that aren't what the should be.
Make sure that if you do change pages then you have a redirect on the old page to the new page or at least the front page.
If you have big site changes make sure that you keep an eye on the affiliate fundamentals. No phone numbers, able to deeplink etc.
Communication: Your business will evolve over time make sure you keep the affiliates informed as to what is selling, what's coming and always offer them help.
Affiliate Rewards: Your affiliates are rewarded well with your commission rates but every so often spend some of your profits on the programme by rewarding them. Whether it's a one month double commission reward or a gift vouchers, be creative and reward your affiliates.
Competitors: Keep an eye on what your competitors have on offer, If you've run your programme successfully then you will have affiliate loyalty but this can change if someone comes along with something that really works.