A lot of different tricks. The most useful ones are:
IF you have a b/w layer with an alpha channel (i.e. a black scribble on a transparent background), repeatedly gaussian blurring and using threshold alpha (in Layer-transparency-threshold alpha), you can make it look organic and fluid, rather than digital.
IF you need a starting point for a picture, use the "flame" plugin in Filters-render-nature-flame and click edit to design your own.
the rest is just colorize, moving, and layer modes