For stiff plastic, like the styrene that Airfix and Historex used for their kit figures, yes, use a fine razor saw. Besides X-Acto and other brands of similar saws, you could have a look at CMK's fine razor saw. Many scale modelers use it for sawing styrene and resin. I think I got mine from Eduard, but I might be mistaken. You can do a web search on "cmk razor saw", though. It has a doble-edged blade, with two grades of fine teeth, and it comes with a plastic handle that you assemble (takes a second to do).

For soft plastic, I'd use a sharp Nr. 11 blade to slice through it. Get a pack and replace the blades frequently, because frequent use will dull them. I use the old blades for general utility work on the bench, scraping seams, etc.

Hope that helps, prosit!