Scoring and snapping drywall is very easy, if you are not near the edge of the board.
Sawing it is pretty messy and planing off the edge is long and messy as well when you just want to trim an inch or so from the edge.
The secret is to have a special drywall edging tool that has two spiked wheels that cut into both sides of the drywall at the same time, with a guide to keep them straight and any given distance from a smooth edge. With both faces of paper scored, a small trim piece can easily be broken off of the edge of a sheet of drywall.
There is a whole video in the Drywall section of the Learning Curve Tab above on Trimming Edges.
This and a lot of other amazing drywall tools can be found by going to speciality drywall stores in the larger cities across the country. They sell nothing but drywall and drywall accessories. Look up "drywall" in the Yellow Pages.