Your input?

Ok, so Im planning on using the S.I.M method for incubation on a few projects soon. And as its been stated several times online, its not exactly a new idea or anything, and people have been doing this and building their own for years. But for me, never using this method, i have ran into a question that I cant seem to find an answer too.
That being... using this method what seems to be the best, or recommended medium to use under the grates? Water? Vermiculite? Perlite? and if one of the last two things... what kind of ratio should you use when using the suspended method? and if with just using water as the medium.. any suggestions on use? Just need some help in usin this method with success.. so that i dont try it, screw up real bad from not having all the points figured out, and kill my beloved little ones! All input appreciated!!

Anyone got any suggestions here?