There are several things you can do to prevent this:
Many soap suppliers sell a low sweat version of melt and pour soap bases. Crafter’s Choice and Stephenson offer a variety of low sweat bases and they sell through numerous distributors. These bases contain a lower percentage of glycerin, to prevent the soap from attracting so much moisture.
If you add additives to your melt and pour bases, this can change the chemistry of the soap, so if you are having problems with your soap sweating, maybe change your additives or leave them out completely.
Wrap your soap as soon as it is set, especially if you live in a humid climate. You can also leave a cool fan blowing on your soaps to dry them out well before wrapping. There are a few different ways to wrap your soaps:
You can use shrink wrap sleeves to wrap your soap. You put your bar into the sleeve,
then using a craft heat gun, or a paint gun on a low setting blow the heat over the bar, ensuring your gun is not too near or too hot, or you will melt holes in the plastic and possibly melt your soap.
Once you have done the top and bottom of the bar, turn your soap around and do the edges.
When you have finished you will have a neatly wrapped bar, that is protected from moisture.
If you cannot get shrink wrap sleeves, you can use cling wrap.
Cut a piece big enough to wrap around your bar,
wrap it lengthways around, then trim the ends, otherwise you will have a large bump in your packaging.
Fold it in then give it a quick blast with the heat gun. Make sure you hold the gun several inches away and have it on a low setting, as cling wrap is much thinner than shrink wrap and will melt easily, risking melting your soap as well. When the layers have stuck together, you can put your soap label on top of the join to hide it,
It doesn’t look as good as shrink wrap, but it offers protection for your soap. It is good for different shapes as well, that are less easy to shrink wrap.
The other thing you can do is store your soaps in air tight containers. This is also a great way to transport them to and from craft fairs, as it protects them from damage.