I used a mixture out of (composted old pine-)bark, vermiculite, perlite,
coarse brick gravel (as used for hydro culture), peat-fibres, long
sphagnum-fibres and (barbecue?) coal.
Actually, it is a very light mixture and I never renewed it.
Avoid peat and calcium. The pH-level should be 7.
I also fertilzed
my plant with liquid Guano fertilizer (bird shit:-) / a very stronh nitrogen
fertilzer) taking half of the recommended amount every half a year.
I also noticed that Nepenthes are *very* tolerable to little water
supply as long as humidity is high. They can be kept much drier than
other CP's almost like ordinary window sill flowers. In the beginning I
killed some of my Nepenthes by providing to much water, so that the
substrat started to rot. (If the substrat smells bad, you should
transplant you plants immediately, cleaning the roots very carefully and
taking new/fresh substrate.)
Regards
Michael