Couple things that I would recommend to help build a customer base
1. Join as many guilds as you can afford that are available in your area. More exposure
2. Show off your quilts that you make and quilt yourself at the guild "show and tells", and let them know you quilted them yourself...
3. Offer to quilt the charity quilts for the guilds, set a reasonable number you can do during the month and offer to do them.
4. Frequent forums such as this one. Post photos of your work, ask any customers you have if they would not mind you posting photos of their quilts when you finish,
5. Join Long Arm forums and such and talk to others who quilt professionally, (this will help you grow your skills)
The main thing to building a customer base is getting out there where the quilters are located (guilds/forums/etc) and show what you can do. When people see you can do things they want done, they will come and ask if you quilt for others. Once people hear I long arm, they ask if I do it professionally (I do not, but wanted the ability to do so if needed) They will come when they see the work you can do, so get what you can do out there.