Wish I had seen this thread earlier...
BeTheBest said a lot of good things here. You need a way to build a relationship with your prospects and customers. Not only is it easier to make special offers to them, but in the off season you can cross sell training material, mobile apps (spend tracking?), and many other things that build and brand your business. side from the benefit of keeping your brand "front of mind" aiding in word of mouth advertising.
if you want to go the coupon route you can add this to your Google Places listing .. this alone will help it show higher in search results.
on the other side, maybe make a "freeware" version that prevent emailing, printing, or filing until the full version is purchased. If I'm not mistaken, you can have your programmers mess-up the "screen capture" so if people try doing an "alt-printscreen" they capture a black image... I think it has something to with blank overlays...
anyway, I do strongly recommend getting a list capture system implemented. aWeber has many strong recommendations. From there choose between the freeware option above, using published coupons, or providing special links in your followup emails that go to special discounted order pages (easier to manage then coupons).
late thing... what usually sells better then coupons is bundling. maybe you can offer people who purchase before X date a free tax guide, or video tutorials for your software, or...... be creative (but relevant!) here.
Hope that helps.