I just use a pair of utility shears or heavy-duty wire cutters. I tried a small hacksaw but it was more work than necessary. The wire cutters are the easiest option just watch out for flying shards of eggcrate.
For fast, straight line cuts, you can knock bars of it out by slamming a pair of pliers or any other metal tool down a line. It tends to send plastic flying, but will break along eggcrate lines. For fine work and trimming, twisting off with pliers works or using sidecutters (wire cutters with the cutting edge at one side of the tool mouth) as noted above is a good idea.