I understand, I owe taxes
I think a coupon could be a good approach when the purchase decision is solely based on cost. My goal though would be to capture contact information so you could email/snail mail/etc them every year. Build a database over time and that information will become valuable based on my experiences. A coupon could be a good hook. I know one accountant who offers to donate $250 to a nonprofit of choice if he can't show how he can save you money. That appeals to morals and is another avenue.
In the end, starting out from scratch I would use facebook to promote the service to audience that is hyper local. You can set a geographic radius as small as a mile. A few dollars a day of boosted posts ($100) a month may get you some leads, and allow you to test the waters with a small budget. Test, test, test. Ads, audience, demographics, interests.
Then when the calendar rolls around ... boost posts and run ads with a higher budget to target the tax season. Practice makes perfect.
Google adwords ~ that's a killer typically with high pay per click costs. But if you can run ads that can convert ... the current advertisers are probably there because of the roi.
SEO ~ target keywords + geo location