Be patient. Those of us who can't say things like "participated in the last war" or "did extremely well the various PvP events" etc. won't be bid upon right away. When other better players are too costly and the generals are trying to save every last card they can, that's when they'll start bidding on less than well known slaves, to fill in their fifth or sixth spot. I'd just recommend going and farming to get all possible rares, and more cards overall to increase your chances of being picked.