The most effective is Word of Mouth followed by great online content.
A client testimonial is much stronger than anything that you (or anyone you hire) can write. This person has experienced your work and can testify as to quality of your instruction.
Online content is great not just to improve your SEO marketing but also to show to prospective clients and existing clients that you know your stuff, that you are more than a trainer but a valuable resource. If you don’t have a blog, start one now in 2015 and write one blog (with at least 500-1000 words) article a month. Your blog, website, and IDEAfit page is your online mouthpiece working for you when you are busy working with your clients or cannot be near your computer. Your online presence has to be uniform across the board. My website blog www.pinpointservices.biz blog matches all of my other blogs and they all say through my words, “yeah, she is knowledgeable and resourceful.”
Hope this helps.