People are attracted to leaders. To attract students, you must be a leader in your martial arts community, or at least create that illusion. Take out front-page ads in community newspapers and in the Yellow Pages. Pay to have a pre-move advertisement at local movie theaters. Project yourself as the best martial arts instructor in the community. If you do everything small, you will be perceived as small.
To attract adults, advertise to adults. Instead of having advertisements showing you performing a spectacular kick or helping a child, show photos of trim, fit adults performing ordinary techniques. Show techniques that an average adult could do with training. Show a lot of tight, firm skin. If ask a group of adults how many of them want to become black belts, few will answer yes. However, if you ask the same group how many want to become trimmer, fitter, and healthier, most would answer yes. So, to attract adults, offer them what they want.
Visit other successful schools and see what they are doing to attract adults and build their businesses. Take what you learn and adapt it to your school. Adapt your adult training methods to fit what adults want and are capable of doing. Average adults cannot do high kicks or jump spin kicks, so stress low kicks and use more hand techniques. Do not attempt to force adults to try something at which they will probably. Instead, motivate them to do what they can do. Adopt techniques from other arts. This may not please Taekwondo purists, but it will put food on the table for your family.