Daryl:
Horizontal cutting wall sections is easy - you can almost snap them off though I would suggest using a razor saw to ease the edges.
Vertical cutting is a different story. Like others I use a table saw. Since I do this for a living I have a Forrest 'no melt' plastic cutting saw blade which makes slicing a breeze. On some walls I need to flatten the backs first otherwise they wobble on the saw table. A sprue cutter and sand paper make short work of this. A band saw would be an alternative to the table saw but make sure you install the correct blade (fine tooth). A razor saw and straight edge clamped to the wall will work but will take some 'elbow grease'. If you don't trust your hand wondering off course try clamping straight edges on both sides of the cut line and sandwich the saw in between.
Joe