(Gee, I can see this turning into the 'How do you cut your roofing shingles Show' ...)
Well, what I do is cut the shingle as I go BEFORE I nail it down. I just lay the shingle in place, make the nick, flip it over and 'score' the backside. Give it a good, decent scoring. With that, you can then kinda gently bend it back-and-forth where it eventually breaks off. Oh, be careful when you're scoring/cutting in that you don't DAMAGE the shingles under the one you're cutting! I usually drag w/me a thin, slightly wide piece of wood to lay the shingle over for the score/cut. That way, if I cut too much I only cut into the scrap piece of wood.