I've had a few problems with stray bits of carbon fibre falling down into the surface of my mould whilst trimming the fabric for an infusion, I think part of the problem is not cutting the fabric big enough in the first place, meaning when i come to trim it in the mould I'm sometimes only taking 1/2" or so off the edge which causes problem fibres to get loose and potentially slip under my first ply.

The best solution is better fabric templates so you don't need any trimming.

Otherwise, press the carbon against the mould as you cut so there is no gap for the bits to fall down. Then you just have to be careful removing the fibres that have fallen onto the flange.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

I have used a light mist of hair spray along the cut lines to keep the fibres straight, spray on, wait for it to dry and cut away.  Leaves a nice crisp line.  the hairspray dissolves in the epoxy and haven't had any issues with strength or seeing residue on the finished part.