There is no magic way to do it that I have found but cold chisel a line where the ceiling meets the walls (assuming they are plaster too) first. I would take the lathe down too but if you choose not to do so, use a small framing hammer so as not to damage it. This smaller tool approach might also be better so the plaster comes down in small pieces rather than huge chunks that could damage your porcelain fixtures, shower door, tile floor? and so forth. Be careful around any existing light fixtures. Wear a mask/aspirator and protective eyewear---a hard hat is not a bad idea if you have one. If mold and mildew have been a problem, consider getting a tank sprayer and treating the exposed space before you seal it up again.
I assume the ceiling is failing or else you would not be removing it just to put in a ceiling fan?