Neil Gaiman once said that as a freelancer there are three things you need to do in order to retain your clients.
1. Do Good Work
Obviously the most important point is to do good work. Do high quality work. Do your best. Strive to be the best in your industry. Provide real value to your clients and they’ll happily pay you money in return as a measure of the value you provided to them. If you can’t do high quality work, it will be very hard to survive as a freelancer. You can get away with mediocrity in an office situation where you are a part of a team but not as a freelancer.