1. Having two fairly large water bowls (one clear water, one salt water) will do a lot to fix this problem, especially if you have a good, moisture retaining cover on the top of your aquarium (or other crabitat container).
In my crabitat, I use a 4-cup plastic tupperware container for the salt water, filled almost all the way with gravel so SuperCrab can climb out if he decides to take a soak. I use a smaller container for the clear water, and the humidity stays pretty high.
2. If the humidity is still not high enough, you can mist the sides of your aquarium with water. This will bring the humidity up, but it won't stay up for very long, even with a cover on your aquarium. 24 hours at most. Also, the spike and then fall of humidity isn't the best for the hermit crab, though it's better than low humidity all the time.
This is the cheapest option.
3. You can make one of your water bowls into a bubbling pool. The benefits of this possibility is that it's still fairly cheap...about $10. It's also pretty cool to watch, and I really love the idea of SuperCrab having a mini Jacuzzi. Makes me quite envious of him, actually.
To do this, you must purchase an airstone, aquarium tubing, and an air pump.
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For me, the airstone was 79 cents, the tubing $1.99, and the air pump, about $7. So the whole thing cost about $10, all available at Petsmart.
The cons of this possibility is that it can be fairly loud. If you bury the airstone at the very bottom of your water bowl, under all the gravel, it diminishes the sound by a lot. But there's still an audible hum.
4. Finally, you can also make your hermit crab a moss area. I haven't tried this, as I was able to keep the humidity up with the previous methods, but it looks awesome in other people's crabitats. This can be done by buying a corner shower caddy, like so:
Stick this shelf onto a corner of your aquarium (this will only work if your aquarium is glass, I think, unless you can come up with something creative) and put some wet moss in it, and the humidity should go up by a couple points.
The moss pit can also be made into a nice area for your crabs if you lean a piece of cholla wood up to it.
Be aware that just any moss will not do for your hermit crabs--spanish moss is not okay! Most people use Hiawatha moss, which can be purchased online at a number of hermit crab enthusiast sites.
Reminder: the target humidity for your hermit crab's home is 72.