remy97, have you tried some of the plugins? For example, my plugin pack http://forums.getpaint.net/index.php?/topic/8318- includes 2 Feather plugins. They defiantly help when making composite images.
You can adjust the amount of the cloning brush by adjusting the "Transparency alpha" level of the primary color:
Try setting it down to 50 before doing skin retouches, for example.
Note for Rick: The text on the help page http://www.getpaint.net/doc/latest/CloneStamp.html should probably be edited to mention this functionality. It is not very discoverable on its own.