We started having trouble with our son around 9 months. We've actually taken to cutting his nails while he is in his highchair. The toenails are typically VERY easy because he can't reach his feet and he is distracted by his finger foods.
For the fingernails, we make sure there are plenty of favorite finger food snacks and then hold one hand up and we stand at the back of the chair. The you switch hands.
I know this SOUNDS sort of elaborate, but it isn't and it works pretty well most of the time.
We have never cut our son's nails while he sleeps and he isn't always a heavy sleeper, so we leave him alone when he is snoozing.
Hope this helps!
Good luck.
--N.