#15 Follow up (and stay in your customers frame of reference)
Follow up! and follow up consistently. Remind your past clients what you offer, see if you can help them. One client of mine went through a court case so had no funds for several months. We cut the project back, I offered to help pro bono, if they kept writing blog posts I would keep putting them up, thus ensuring they maintained return on investment. You think that client is ever going to leave me?
#16 Think Long Term
Clients are for the long term. Do not rip them off, burn their business. Repeat clients pay more and earn you more due to efficiencies.
#17 Clients as your sales team
Clients are your best sales people, so look after them, obsess about their business, recommend their product to others.