What's the right way to sand/shave/reshape particleboard?
If you don't have a lot of material to remove, you could use a belt or palm sander to gently sand it down.  Be very careful not to press too hard so you don't damage the wood, if you take your time it should work just fine.

If you have to remove a bit more material you could use a circular saw, just make sure you get a saw blade with lots of teeth.  For materials like this the more teeth the better, as it will give you a smoother cut and minimize chipping.