I use cactus soil and then I mix in pumice, a little compost, akadama and kanuma soil. Akadama and kanuma I found at a Japanese store that grows bonsai, it makes it easy for me to visually see if the soil is still wet or dry. Akadama and kanuma gets into a lighter color as it dries out, plus it is quite coarse so as I mix it in with cactus soil, really makes it quite well draining.
But pretty much cactus soil with added perlite and pumice will work quite well too.
