Lots of good ideas in the responses so far. I think the most important thing in a marketing campaign is that it be a campaign, not a just a flurry of activity when business is slow. The way most single person businesses work is that they do a lot of marketing, get a gig, stop marketing to do the gig, and then there's no business in the pipeline because of the hiatus in marketing.
If you're in the US look up your local Small Business Development Center. It's a program of the Small Business Administration that offers FREE business counseling to small entrepreneurs (less than 500 employees). They can help you develop an ongoing marketing campaign that fits your business. Then, of course, you have to execute that.
A plan of personal networking such as Chamber of Commerce mixers, business leads groups, social media, carefully chosen charitable groups is very important. As you know it's all about eyeballs: the more eyeballs that see your work the more likely there will be a customer among them.
The key thing about any marketing campaign is, of course, a commitment to a continuous marketing effort, even when you have business and especially when you don't want to.