First off, I recommend picking up Google AdWords, not just Yahoo. It basically comes down to usage, and Google is the clear market leader, unless you are targeting China, and then Baidu may also make sense.
If you do any trade mag advertising, try eliminating several months of print and re-investing in PPC (Pay Per Click) advertising.
Unfortunately, similar to a print ad, you can not gauge your results until you try it. Fortunately, if no one clicks on your ad, you will not be out a sizable capital investment.
But, from my experience with clients in many different industries, PPC and SEO (Search Engine Optimization), or natural search, is the best way to advertise your Web site.
