As Cayuse said...it's word of mouth. So, if you had clients who don't do much with their horses but have them in their backyards and trail ride every now and then and don't have "connections"...these can be dead ends for your business.
So, you'll want to get clients who have "connections" who are active in horse competitions, or clubs, groups, at a barn, etc...
Ideally at some point I would like to become a resident trainer at someone's ranch, but don't know really how to go about doing it.
This is a good idea, but be sure to find someone who has the same phylosophy as you.
1. What do you want to train in? Western or English? Competition? Or just basics? Find the trainers near you who have a barn and do what you would like to do.
2. Call and ask about becoming an apprentice, an assistant, a groom (for dressage), etc.... Then set up a time and just go. They'll see your skills and you'll see theirs and you'll both see if it's a match.
I am also making a website, and ideas of what you would be looking for when looking at a trainers website would be much appreciated.
The website is a good place for someone curious about you to see what you look like, find out about your training ideas and your accomplishments, testimonials are good for websites, too. Pictures are great, too.
If you can, go to the local tack shop and ask if you can do a demonstration in their parking lot.
Join a horse club of some kind, or contact several local barns and ask them if they need a trainer.
The more people who SEE your work, the easier it is for them to ask you for training.