A few things:<p>Marketing to existing customers. Is there a new product your existing customers also wish you made? Is there anything else they would pay good money for you to add on? Look for competitors that do what you do and see what else they offer. See if they've given any clues on their website to what else their customer's like. Heck, find their customers and market to them. Do a bit of competitive intelligence, see what else customers want, and confirm it with your customers. Its never too early to get a whole new product line in the works.<p>Market through existing customers: Call up all your biggest customers and check in and see how they're doing. It works especially good if you have a new feature you've added on to their subscription that you're announcing to them. Encourage them to use it, then say "We're looking to expand and grow our business, do you know any other companies that might be interested in our products?" Don't pressure, just ask, and if you get a name, follow up with them. Don't use the existing customer's name without permission, but ask if you can i.e. "Bob from Wigit-MakersPlus suggested we call you..."<p>I would do basic advertising research. Even if you don't decide to advertise. Think, where would you go to advertise? Who are your potential customers, and where can you find them? Who do you contact at the company to get a sale? I wouldn't discount online advertising, such as feed sponsorships or niche websites. If there's a blog or site many of your customers go to, find it. If it doesn't do advertising, contact them and ask to be a sponsor, you might get a great deal.<p>If every single customer is profitable, expanding your business involves getting more customers, but like someone else mentioned, increasing the profit margin on each sale is important as well.