Sandpaper, or use the nail file on your SAK which requires you to open it first and file your nail down so it no longer is broken or cracked which is a structural weak point. If you don't have either handy a brick or sharpening stone will do the trick.
I have an imperial knife which was a "nail breaker" till I managed to clean it all up and oil it. After breaking the nail a few times and having none left to use I had to resort to using a pair of pliers as I probably stab myself with the knife had I used my left hand to open it. Crappy thing about that knife in order to test if I cleaned it up and oiled it and got it working right I had to close it and reopen it, which meant going through that problem again. (And no I am not complaining about a broken nail, I just adapted as I could no longer open the dang thing with my dominate hand safely till I fixed the problem.)
And a heads up don't EVER tell you buddies in life that your looking for a tool to help you open a knife because your worried about breaking a nail. Guy or girl there is no way around it looking embarrassing, just buy the tool mentioned earlier and use it.