I second the advice on the scissors that John recommends. Very useful.
I use a modified technique for cutting straight lines borrowed from marking joinery in furniture making.
I put the knife blade tip on the cutting line then slide the straight edge up to the blade and note the distance and relationship between the straight edge at the knife blade and the cutting line, then line up the straight edge with the rest of the cutting line, maintaining that same distance and relationship all along the cutting line.
You then cut. It is often more effective to make several passes to cut instead of trying to cut in one pass.
