Psychopsis favorite medium?

[piaffe97]

I believe I need to repot my psychopsis. It's been blooming beautifully for about a year, with plump white roots poking out of the slits in the pot. Recently, it's stopped blooming, and the roots that were previously healthy, are now greyish brown and dry looking. Nothing has changed about the plants environment at all! I suppose the next logical step would be to repot into fresh medium, but I'm not sure what to buy. Any help would be greatly appreciated!!!!