Great thread. I am a novice and my welds look pretty crappy but I just grind them down for aestetics.
Just a few tips from a novice:
If you are welding multiple pieces or one piece in many spots, begin by "tacking" the pieces. That is, spot weld in the places where you will be welding. This helps hold the pieces in the postions you want with minimal warping.
Bad looking (not structurally bad) welds can be cleaned up with a grinding wheel so that they look more pleasing to the eye.
If you are mig welding make sure your work is grounded with the grounding cable and that you have appropriate eye protection. Don't go blind being stupid.