I saw a similar thing on the Make blog a while back, so I decided I'd try it with acrylic!
I first tried it on a sample of frosted acrylic, and then on a larger sample of black acrylic.
The technique is pretty simple, for these trials I'm using soft wax crayons. I heat the acrylic on a hot place to around 80°C and place wax fragments on top. Once they've melted I use a palette knife to spread the wax around then paper towel to remove excess wax.
It doesn't stick very well, so I've been trying to find a harder wax to use, but the crayons do work and are cheap and easy to work with :)