You need to drill down on who your typical customer is. What sex are they? What age are they? Where do they live? What is their education? Once you know the exact target demographic of your typical customer, then you can easily go and target them more easily on sites such as Facebook, etc.

Are there any other sites with an audience similar to yours? Try to find them and see which ones are sending out newsletters - they should have a list. Just contact the site owner and ask if they would be willing to sell you a Solo Ad or mention you in their newsletter... It should work well for you.
Good luck
Art

Post on forums in your niche. The weight gain/body building niche has some big forums, and if you become active, you can get some good, steady traffic from them.

Also, consider reaching out directly to gyms. See if they will let you post on their site, or better yet, place some sort of flyer on their front desk. Offline marketing is powerful, and often overlooked.

Michael

I would turn your syndicated articles into 3-4 minute videos that target your keywords - you can pick up some very good traffic on youtube with concerted video campaign. If you have some highly profitable PPC campaigns where you have a good grasp on the ads and keywords that work you may want to try taking your marketing offline with such ads

Jeff

I must congratulate you first, because starting getting good business from a 6 months old site is really a good indication to become rich soon.

You can try following without much effort

1. It appears that you have setup GA that tells which channel sends you how much traffic. Have you setup tracking to measure the source of your inquires and sales. If not you must do it. Then it will be water clear to you where to put more effort.

2. Try to catch the lost sales. Setup automated follow up emails against your inquires and unpaid orders with your personal details. This will close at least 20% of the lost sales.

3. You can try referral / repeat purchases from the existing customers. You should keep in touch with them. You can easily do it by setting up automated emails (say after 3 days, after 2 months etc), just telling him how he is doing and refer your product to his friend.

Hope it will help.

I'd imagine the weight market is pretty competitive whether it is gaining or losing weight so PPC may be too expensive unless you can find niche keywords. Have you tried content marketing alongside your article writing - maybe develop useful infographics, images or videos as jbsmith suggested but tailor these to your audience.

WillR is right - you need to have a really clear idea who your audience is - how much detail do you collect on your users so far - can you mine your database to identify key segments and match this against spend etc?

Research is also a great way to learn more aout you existing users - incentivised surveys (Survey Monkey, Survey Gizmo etc) can help you get even more information - what websites do they visit, why the buy your product/service, what they like most, what they don't like etc. This can be really helpful in developing content to reach your audience which you can promote through your blog and social channels.

Good luck and sounds like you've done a great job getting to this stage!

Facebook still has not killed all communities out there
there are forums about every topic imagineable.
But also other types of communities.

You can get active on these sites.