I have tried to keep hermit crabs as pets at various times in my life, and each time the poor things have died during their first molting process. I know that they are incredibly vulnerable during this time - what can I do to help them get through this process without harm?
Diet: Make sure your crabs are getting a good diet pre-molt. A varied diet including lots of high-energy and high-calcium foods. Natural peanut butter, honey, cuttlefish bone, and sesame seeds are all good choices. Fresh or dried fruit and vegetables or even unseasoned meat trimmings can be added to this to help round things out.
Humidity: 80% RH is recommended for most species of hermit crabs. This is especially important during molting. Certain species, such as the strawberry hermit crab from Australia, require even higher humidity. 90% RH is usually sufficient for them.
Substrate: It's important that hermit crabs be provided with a substrate that they can burrow into. Again, this is especially important during molting as surface molts almost always prove to be fatal. The substrate should be light and loose enough for the crabs to dig in, but should also be able to hold its shape so that the crabs' tunnels don't collapse in on them. A smooth, round-grained sand, such as play sand can work well. Add enough water to make it "sand castle" consistency. EcoEarth (shredded coconut husk) is also an excellent choice. I find that it holds moisture better than play sand, has a nice texture, and helps keep the humidity in the terrarium up.
Temperature: 80 °F (27 °C) is generally a good temperature to shoot for. Strawberries tend to like it a bit warmer (85-90 °F / 29-32 °C). You can achieve this with a UTH (under tank heater) and/or a heat lamp. Try to create a gradient within the terrarium to help the crabs thermo-regulate.
Other than that, try not to disturb your crabs when they're molting. Don't stress them out. And just leave them be until they re-surface on their own. Sometimes older, larger crabs might disappear for quite a while, several months at a time. With any luck, they should re-appear just fine!
Water: It is important to provide water for hermit crabs. The dish should be deep enough for the crabs to submerge themselves in, but shallow enough that they can easily climb out. Tap water is fine to use as long as it is dechlorinated first. Use a crab or reptile formula to dechlorinate as most fish/aquarium dechlorinators contain slime coat and other additives that could be harmful to your crabs.
Ideally, salt water should be provided as well. This should be made by mixing dechlorinated water with a marine salt formula (eg. Instant Ocean).
Both fresh and salt water can be prepared in bulk and dispensed as necessary. Providing a dish of each allows your crabs to regulate the osmotic balance of their shell water. However, some people prefer to give their crabs weekly salt water baths rather than keeping two water dishes in the terrarium with them. Both approaches seem to work.