My potting mix consists of long fiber sphagnum moss in the bottom of the pot followed by a mix of sphagnum peat moss with pumice rock mixed in for drainage, then a top layer of long fiber sphagnum moss. A good soil mix should retain some moisture and definitely drain well. Nepenthes do not like to be kept wet all of the time. Other common adjuncts and soil mix range from coconut husk, charcoal, lava rock, perlite, orchid bark, a type of cedar chips (I hope that's correct), limestone, and straight New Zealand sphagnum moss. It seems every grower has their own recipe for success. So whatever medium you use, just make sure that it is a "loose" mix and allows for good drainage and aeration. 

Sphagnum peat moss is cheaper and easier to get than pure long fiber sphagnum moss. The downside of sphagnum peat moss is that it takes longer to hydrate than long fiber sphagnum and falls apart more when repotting thus possibly disturbing the roots. A good healthy soil will look "good" and not be black or smell. With proper lighting and humidity, the moss will be green and look alive. For me, grasses invade the pots but the Nepenthes don't seem to mind. When the grasses are looking good, that's a sign the the soil is doing well and doesn't need to be replaced. When one of my Nepenthes appears to be suffering from the "creeping black death" or the "graying syndrome" I'll repot with fresh soil and hope for the best. Other than that, I'll replace soil when I'm repotting to a bigger pot. 

Nepenthes are very adaptable to a variety of soils. Every grower seems to have his or her "personal" recipe. So if you like the quality of plants you're receiving from your vendor why not try what recipe they are using. Oh, and don't use that decorative Spanish Green Moss stuff. I've been told that can be death to carnivorous plants. Long fiber sphagnum can be bought from Cal West Orchid Supplies. They deliver quickly, reasonably priced, and have many different sizes of sphagnum moss available.