With this type of thing, referral is always the best method for marketing.
The best way to do this is:
1) Start small by leveraging your current network like people you already train
2) Do everything you can to make sure you your clients have a GREAT experience
3) Give them a means and motivation to refer
A means to refer is obviously word of mouth which you don’t have to provide, but try creating a facebook page or a short website with your training services info and share this link with your clients. An email to a client asking for a referral with a link to your site can go a long way.
Then, you can provide additional motivation – for example, for every new paying member a current user brings they get 2 free training sessions.
Hope this helps and best luck!