It's always been easiest for me to trim my babies' fingernails (and toenails) while I am nursing them. I just make sure that their heads are propped up sufficiently using a boppy or a pillow (or the arm of a big recliner) and I trim away while they are busy nursing. While I'm trimming I usually say "It's your job to be very still and mommy's job to be very careful". After I do it this way for about a year I don't have trouble trimming their nails even if I'm not nursing them at the same time. When they start talking they can even say the instructions along with me. I've done this with all four of my babies and I haven't drawn blood yet. ( I hope I haven't jinxed myself!)
Check out other tips at Shannon's blog, Rocks In My Dryer.