I do a heck of a lot of scratchbuilding, in fact, most of my modelling seems to be scratchbuilding or kitbashing of some sort.
What I have found in all my years of experience is that firstly, plastic quality matters. Plastikard sheet for instance will give a clean, non-grained cut, where as Evergreen sheet will grain. The grain tendancy is useful for simulating wood, but it also means that the snap-cuts are never straight, and require sanding which is difficult on small pieces without sanding your fingertips off.
For anything below 1mm styrene, using a ruler draw your line in pencil. Use a craft knife (not xacto) and cut down hard, keeping your eye on the line just in front of the knife... you'll find that the knife blade will automatically follow your eye. After the first hard cut, do a couple passes, and a hard cut at the ends of the line (since these tend to be shallower), then snap.
For 1mm, use surgical or sharp scissors to cut pre-drawn lines. For fine strips or detailed areas, use a dremel to cut apertures (holes) and a craft knife using the above technique.
For thicknesses greater than 1mm, use a craft knife to start the cut, then use a hobby saw to continue it, then snap it off and clean up the edge.