Can't downshift rear derailleur I'm trying to fix my wife's bicycle which sat out all winter.  I can't get the rear derailleur to downshift from the twist grip shifter.  I have inspected and cleaned and lubricated the cables, nothing is frayed or stuck.  I can move the derailleur with my thumb quite easily, and if I hold it I can move the shifter.  I can also upshift, but I can't get the derailleur to downshift from the shifter.  Any ideas?  It's a Shimano system.
I solved a similar problem by dousing the inside of the shifter liberally with WD40 and leaving it overnight, I mean really soak it until it's dripping. I had to do this twice but the shifting was perfect afterwards.