Doyles dart den has great information on them. As far as I know they only culture in flour. My only problem with them is that they don't produce fast enough. They are an excellent supplemental food, especiallly for non-thumbnail frogs
We have always used 2 parts whole weat flour, 2 parts white flour, 1 part brewers yeast. I keep my cultures in shoeboxes with the lids on. I get larvae out every two weeks in the warmer months and every 3 weeks in the cooler months.
One person I know added a good portion of potato flakes to his mix. He clais that this makes them reproduce much faster...extra fat/protein maybe? However, whole grain oats contain more protein than potato flakes(at least the brand I'm using).