Whenever I cut my fingernails (which I do pretty frequently on my left hand) I take a metal fingernail file (I hate ebony boards) and smooth the edges that the clippers make. After I do this, I file my callouses, so that they smooth out with the rest of my fingertip. This prevents the place where your callous meets with the uncalloused part of your fingertip, where the callous may poke up a bit, from catching on frets when sliding or just moving your fingers near the fretboard. Filing and then applying a little bit of hand lotion is a great way to have soft, smooth skin, but you still won't hurt when you play. I also personally file my knuckles. It's just like the kind of training where people hit metal or wood over and over again to strengthen their hands, but without much work. Once again, though, APPLY LOTION after filing. If you don't, your skin will become dry and hurt more.