Any old free draining crap for me. It usually ends up being bits of whatever I have lying around for the fussier plants in my collection, plus a load of vermiculite mostly because I bought too much in bulk. As already mentioned by many, I also chuck in some nutrient added compost, too e.g. multipurpose, John Innes etc.
Think about where they naturally grow. Some grow on shear rock faces where they will collect wind blown organic detritus (moss, leaves) while others are epiphytic on moss and some in leaf mould. For habitat photos, checkout Fernando's (and others) excellent adventures at
http://www.pinguicula.org/pages/pages_prin...%20field%20trip
I think it highly likely that the availability of nutrients will effect flowering and division habit as well as growth, as with most plants.
Most critical is getting water/humidity and temperature/sun correct related to season. (Humidity will usually be greater for organic media watered by tray method.)