My specialty is technology. That depends on your business, but the traditional best way is through networking with others. If you're looking for an online presence, Shopify is great for easily building a website with payment processing; it's specifically meant for small to medium sized businesses. If you're looking for online advertising, a form of scaling your audience, look into Google's adsense service or Amazon's. Links are below.
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Figure out what full capacity is first. How many clients can you handle in a day if you were fully booked with your current staff? Figure out the revenue you would generate. Then look at the figure you are looking to grow to. How many more staff and clients will it take to reach that number? Also calculate any subsequent increases in expenses. Once you have figured those things out then focus on you current marketing methods and make a chart and listing what has worked and what has not. Take your winners and create a budget to duplicate those efforts. I would caution against new marketing strategies with out assistance or someone that has experience as you may find yourself losing money trying to work with a new marketing strategy. Good luck!