Hi,

What is the best way to mark cut lines on pre-preg carbon fibre, to avoid making cuts on the mould.

Some guys at my place use white China markers but I find they aren't very precise, due to being fairly blunt.

Cheers
Steve

Not sure if you mean scribe lines or marks for trimming material...

For scribe lines thin fine-line tape or one edge of that adhesive teflon (Tooltec) or thin flashbreaker tape is nice for leaving a hard edge to trim to in the finished part.  The teflon (with yellow backer) is nice because it peels off more easily than most things after a cook.

I have used thin (1-2mm) sheets of HDPE or similar hard plastic to trim prepreg ply edges and laps in the mold.  You mark the cut with a slip sheet of backer in there and then peel back, lay the (pretty bendy) piece of plastic right in to cut against.  Works best with woven but ok with uni if you leave your backer on.  

Most materials you can get a black or blue biro to show up fairly well
If you want to go the "crayon" route use verethin 
They are solid pigment not wax based like chinagraph. Gold (no99 I belive) shows up well,you can sharpen them like a normal pencil