Neps, in general, need an open, airy soil media, slightly moist, with relatively warmish temps, and ~12 hours of light per day.

here a website that has good care sheet for nepenthes (http://cobraplant.com/index.php?main_page=page&id=32)

You may also consider purchasing a small water sprayer.(they're about 1$ at many stores) Spray your nepenthes with purified water in the mornings and in the evenings to increase humidity. these type of plants thrive from high humidity and warm temps. simply spray its leaves along with pitchers. never allow the soil to become too wet or too damp. as petiolaris mentioned, Simply moisten. Nepenthes are good house plants in shaded areas. morning sun is fine for these plants, never leave them outside in the hot sun or its leaves will burn. hope this helps.

Spraying plants does NOTHING to help humidity. You'd have to spray every 10 minutes 24 hours a day. They dry off too fast. Don't worry about humidity.

Here's a pic of my nepenthes when i barely recieved it from Cooks. When i obtained it, it was just a cutting from a larger plant, so i was told in my e-mail. it was pre-potted. kept it in a terrarium for 2 months but it did not seem to grow as vigorous as i intended to grow them/ growing CPs in terrariums is not really that neccessary, however, some do seem to like it like Pings. but they also do better in windowsills along with Neps.

Here's a pic or My nepenthes when i used to grow it inside a terrarium, i just barely watered but i did not spray it with water at all. The nep is in the far left view.

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After a few days, i've read that these plants like high humidity and warm temps in their wild habitats. so i tried mimicking the same environment for this plant/ so i took it out of the terrarium,placed it in a windowsill.,watered it atleast 3 times a week(not the tray method)
it showed some signs of improvement..
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i began spraying this plant with purified water twice a day , early morning, and night. then.. big pitchers began forming and the plant just erupted with larger leaves. very nice.
pic one:
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pic two:
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chicchan your light was too far from the plant when in the terrarium. What type of light was it? Mylar works better than aluminum foil. Also the pot was too big to start with. Pot size IS important. How long have you had this plant? It looks very good and healthy.