I don’t draw so well, and when making these comics, I like to cheat as much as possible. For example, I use a tablet PC with an active digitizer so I can draw directly on the screen and use control-z to undo. I like to draw with MyPaint (at least until the new Gimp, with advanced brush dynamics it available, stable and plays nicely with my Ubuntu desktop). MyPaint has layers and opacity so I can load an image and trace over it, but it doesn’t make it easy to have more than one background image, or to adjust their size and position.
Enter Compiz desktop graphics system and, in particular, the Brightness, Opacity and Saturation plugin — accessible from CCSM — which lets me adjust the opacity of the whole MyPaint window with the mouse wheel and a modifier key. So now I can quickly trace over whatever is in the window underneath.