The easiest way to get and maintain calluses is to practice daily.

If strings are cutting your fingertips one or more of a few things should be looked at.

a) Have the action on your guitar/banjo/mandolin, etc., checked.

b) If you haven't changed your strings for a year maybe the trebles are corroding and when you dive into that rock solo with a slide from fret 0 to fret 15 and stretch it, all in a trice--well, look out. May as well run a hacksaw over the ends of your fingertips.

c) A string that can cut skin can cut calluses, which are just dead skin and deadened nerves. You'll find that in winter the tips of your callused fingers will freeze very easily, and you'll also find out how deadened your nerves are when your fingertips thaw. Hurts like a sonuvbitch.

d) The nails on your chord hand are too long for standard tuning. That could cause your fingers to angle down, thus having that thin line between callus and 'live' skin exposed to a string and slit-ouch-&%$#@^& - #*)&%^$%^@# it goes.