Trial and error.
I've been selling since 98 and it's all still trial and error. (four eBay IDs which two are retired right now)
I still tweak my auctions with different keywords depending on fads, time of the year, changing template etc.
Every six months or so I pay for a one month subscription to Sellathon which shows me how customers find my auctions, key words, demographics, time of day, whether they bookmark auction, how many searches come directly to my store and how many via random searches on eBay, search engines etc.
All of these facts differ widely seller to seller. So it's hard to narrow down what works for me and my several IDs as versus your product.
I would highly recommend starting your own ecommerce site also and getting a free ecommerce store via ecrater.com.
I'd also highly recommend joining selling groups, ecommerce groups to be with others that are serious about successful online sellers also.