In addition to lighting and sampling, resolution is your friend! I usually go for 2000x2000px minimum and downsample to the image size. It's not that different from working with pictures from a digital camera. The renderer has to calculate a color and value for each pixel in the image, and it works better when there are more pixels. Also, when you do get some grain in the larger image, downsampling helps eliminate the artifacts.

As long as you're here, Ilona, I'd also add that you do beautiful work :)