'arms up' creates an artificial 'perkiness' because it pulls the skin above the breasts upward.
sort of a 'boob lift' effect.
the arms-up pose also works well with an arched back, which causes more projection & perkiness.
the difference is fairly stark.
with stage 1 ptosis (what plastic surgeons call a normal breast) slouching creates a fair amount of droop, while arms up & back arched creates a very dramatic perky-busty look.
the laying on the side pose is also a classic, since gravity creates cleavage very well in this pose, and the waist-hips ratio is exaggerated since the ground-side is being forced flat, the up-side of the body shows all the differential from both sides compounded. sort of a "double curves on one side" effect.