How do I easily separate unglued PVC pipe from joints?
The best lubrication I've found is some good ol' Elbow Grease™.



Actually, you should measure the proper lengths and just cut. Then glue it up and install. You can mark on the PVC how far it should go into the fitting if you want to be really precise. However PVC itself expands and contracts a lot with temperature change and is fairly flexible anyway. So, in my opinion,  being extremely precise doesn't mean all that much anyway. Also, The solvent melts the PVC surface to weld them together, so the fittings will likely fit differently while gluing, thus making the dry-fit method even worse than just measuring.

Another solution could be to mark how far in the fitting the piece should go and lay them next to each-other in that position, even tape them together, to check your layout. That seems pretty time-intensive, though. I still think it would be better to just measure accurately and double check your measurements.