Heat plays an important part in the molting cycle of the hermit crab. A simple UTH (Under-the-Tank-Heater) or heat lamp will suffice to maintain a temperature from between seventy to eighty degrees, and a humidity rate of 40%. These are conditions very much like our tropical friends' natural environments. Most hermit crabs sold in the United States are from Florida and the Caribbean.
Maintaining an environment that's mostly enclosed (an aquarium with a cover, for example, or a plastic container with plastic wrap over most of the holes at the top) is essential. Wire cages and containers without lids don't lock in the necessary heat and humidity.
In the molting cycle, heat and humidty help a hermit crab maintain the level of moisture needed for a successful molt. Moisture must collect between the new layer of skin and the old layer; the pressure of the moisture will eventually cause the skin to crak, which begins the molt. Moist outer skin is easier to shed; a moist skin underneath helps the old skin slide off like a glove (only with a lot more work).
My own tank is quite jury-rigged. It's equipped with a cardboard lid that fits across the top, leaving a corner open for the desk lamp I use as a heat lamp. The bulb I use is a 75-watt "moonglow" bulb. I covered three sides of the tank with aluminum foil, theorizing that this would help reflect the heat back inside rather than allowing it to dissipate. My crabs have the added challenge of surviving definitely non-tropical dry and cold Ohio winters. I use a "moonglow" bulb because this is like a blacklight--it does not shed much light. Hermit crabs, being nocturnal, prefer near-darkness. This way, they're not scared to go out and eat and drink or just climb around, as they might be under a bright heat lamp.
I use a heat lamp rather than an undertank heater today because of an incident years ago. I had an UTH positioned just underneath the waterdish, because the heat source would make the water evaporate and therefore create more humidity. Pre-molting hermit crabs often dig under the sand of the waterdish; if the sand is moist, this will create more humidity. A good thing, or so I thought. I had a small Ecuadorian crab named Mercury who was digging in and out on a daily basis. I went to Cleveland for a few days, leaving Mom in charge of the crabs, not worrying because Mercury had always dug herself back out. When I came home, I found Mercury under the waterdish, dead. She hadn't dug herself out this one last time. The heat level under the waterdish with an UTH underneath it is too hot for hermit crabs. I read somewhere online to do this to increase humidity but--let me tell you, in honor of my Mercury--DON'T! If you must use an UTH, place it elsewhere. Not under the waterdish.
To increase humidity, try adding a few extra waterdishes. They will evaporate, putting more moisture into the air. Also, try putting some natural sea sponges in the waterdishes. More surface area = more water = more humidity! (As you'll see in my 'Food' section, crabs can eat these sponges and get extra nutrients--so make sure you use NATURAL sea sponges. You can find them online at petdiscounters.com, or on eBay. Many pet stores carrying FMR products and crabs also sell small packages of natural sea sponges.