One thing that hasn't been mentioned yet:
You should always work at a much higher resolution than you plan for your final image.
For example, if your final image will be 800x600 pixels, you should be working at 2400x1800 while developing your image. Then, when finished, resize your image down to the final dimensions. Along with feathering as you create your image, this will remove much of the "jaggies" you're seeing.