Keep it covered with a lid so as to avoid loss of moisture. If you were to melt the soap, let it harden, remelt it, let it harden again, and so on, eventually the soap would loose its moisture become flaky, waxy or crumbly.
Some people use a microwave with success.
Bring the water in the double boiler to a boil then remove from heat. If your soap isn't entirely melted, repeat the process. The trick is to not let the soap get too hot - this will reduce the transparency of the clear soap. Visit our Supplies page for detailed information on these products. Don't add it directly to the entire batch of soap or you will get clumps of color.