I use a modification of the method mentioned by
@JAZ
if the bread is crunchy w a thick crust and the sandwich is thick and has soft filling or slide around fillings :
I cut the top in 1/2 while is is alone on the cutting board by itself , then put that on top of the completed sandwich and cut right through
the rest. no flipping.
I did not invent this myself , unfortunately .
I saw it as a mail-in " Tip ? in Cook's Illustrated many many years ago.
at the library of course !