Corina, I think your sponging is beautiful! I answered the question you post on my blog in a new comment on that post, but thought I would add it here so you don't have to check back over there.
I use Gina K Pure Luxury White for most of my sponging. A very smooth cardstock, like Gina's, or SU Whisper Whiter, or Neenah Solar White seem to work best for me. I also like to start with the lightest color. I tap off extra ink at first, on scratch paper and and start with my sponge dauber off the cardstock, then swirl it onto the paper in a circular motion. As the color builds up, then I switch to a darker color.