comparison ko lang between my old and new plasma:
panasonic 42pv70 - more than 3yrs old na, it is VERY resistant to burn in, kahit watch ng 2hours movie with black bars or static image, nawawala agad burn in after changing to other channels, same with gaming on the ps3, it really takes a long time for burn in to occur. Consider that this model has NO ir prevention features whatsoever.
lg 50pk550r - i estimate my break in of this recent unit to be more than 200 hours, mix of v120 slides and full screen viewing, i notice it is still prone to burn in particularly gaming and static logos. BUT compared to when i got it new (less than 20hours used) malaking improvement na ang burn in resistance nya. I think it has a long way to go to be as resistant as my old 42pv70.
in short, just use and use and use it, it'll become more resistant with age