I tried once. They don't like to be held down like that so I gave up. I put lava ledges and a pee rock in their cage and that seems to help keep them at bay a little. But I frequently have little scratches on me from them climbing on me.
I was getting little scratches too, so I actually trimmed Charlotte's nails today with my mom's sewing scissors. I tried the nail clippers first but they didn't work as well, and the pet nail clippers didn't give enough visibility. However, the sewing scissors can be very sharp and pointy (my mom's aren't as bad as many pairs), so this probably isn't a method that can be applied everywhere.
It helps a lot if you have a rat who will hold still; Charlotte didn't give me any trouble at all. If yours won't hold still for the clippers, you could try getting someone else to hold them while you trim.