When researching Renovation 3rd Edition a few years back, came across this nifty trick for aligning mosaic tiles, courtesy of Wayne Brasher. Mosaic tiles come in sheets, adhered to a paper backing. Once you set tile sheets in adhesive, the paper backing starts softening almost immediately. Using your palms to slide sheets into alignment just doesn't work because tiles come loose and everything's soon out of whack.
Instead, do as Wayne does: place the straight edge of a trowel against each row and tap the trowel lightly with a hammer handle. A rubberized handle delivers a softer tap.