Great question! I assume the listing owners (the folks paying the monthly fee) are the trainers? Here are my thoughts, in no particular order:

1) I googled FitnessSpot, and found a large number of results. I'm not sure if FS is a website, or a local club in Michigan, a studio in NYC, or something else entirely. A little SEO will go a long way; along with some brand building activities. Get your name out there in multiple channels, make sure you dominate the search results.

2) I'm assuming you're a startup. At this stage, the name of the game is users. If you have a large user base, trainers will pay to get access to them. If you a lot of trainers, users can still use the phone book or google to find them. The key is user enrollment; and that's driven by traffic.

3) Is your value as a matchmaking service for trainers and their clients? That seems like a one and done activity- client meets trainer, trainer meets client; they both leave the site. There are a couple of options here- you can continue down this track, which is fine; but looking for either trainer or client to be a long-term revenue stream seems....limited (at best). In this case, I would recommend exploring other revenue streams - maybe adsense, maybe affiliate marketing. In any event, the name of the game here is still traffic & brand awareness. Think of a comparison to a dating website- by its very nature, if you are doing a good job, your users will fall out of the marketplace.
You could also look to drive more engaged users. This is likely my recommendation (without knowing many specifics). In this case, you'll need to think of other value you provide. Is it tips & tricks, nutritional information, exercise of the day, etc.? The point is that you would need a much smaller user base to drive the same amount of traffic. Your efforts can be less focused on relentlessly attracting new users, and more focused on building an online community. It's under this model that you'd be more likely to monetize the users/trainers directly; but keep in mind what they're paying for- users will pay for valuable content; trainers will pay for access to the users.

I'd love to discuss this with you in more detail, and better understand if I can help. Please drop me a line at your convenience to chat. Cheers!