I use a plasticine that is soft. At least it is soft here where I live (Southern Mexico at sea level). One thing I do is trim my fingernails regularly. And I find that fingernail damage is easily repaired. But after years of doing this I must have learned to keep my fingernails to myself.
I have developed a number of tools to work the plasticine that don't have fingernails, mostly made of wood. Wood seems to work better with plasticine. Most of them I have carved from tongue depressors and popsicle sticks. And brushes. I use brushes a lot. I wipe them off with a rag frequently while working with them. Little artists brushes. The stiffer ones are the most useful. And then to finish it off I use naphtha and a softer brush to smooth it up glass smooth. That is, if I want a smooth surface, which I appear to be addicted to. I sometimes wish I liked textured surfaces. So much easier to mold and to cast. But who wants to look at a pretty young woman with a textured surface? Probably someone, but not I.