The basics:
Filter menu>Render>Clouds
Filter menu>Gaussian blur> (20% or higher)
Filter menu>Waves (play w/ No. of generators and Wavelength)

Options are endless...add solid color layer above below, play w/ blending modes...repeat wave filter (CTRL+F)...add gradient map adjustment layer, play with blend modes...in the wave diaog, change from Sine to Triangle or Square...