Increasing sales of my Print on Demand stores

I'm selling my designs on several Print on Demand stores (Redbubble, Zazzle, Teepublic, Society6). I would like to increase my selling. I'm ready to give 40% of the amount what I earn. The seller will be free to choose the products for selling from my stores. Until now most of my seling where in USA, Australia, Germany, United Kingdom, but it is not important in which countries products would be sold in future.