How can I stop my shifters slipping when climbing? I'm having trouble with my rear mech and/or shifters. Under heavy load (say, when climbing) the rear mech is slipping. The two photos below show my shifters before and after a climb. I've not touched the gears during the climb, the shifter slips and I just change gear.

What would cause this? I didn't think putting power through the drive train would push/pull the mech.
I don't know a great deal about the old shifter types, but two thoughts come to mind - 
1. Very obvious, but have you tried tightening them to increase the friction?
2. Might there be grease in the friction surfaces?  You could try opening them and cleaning them with deagreaser/brake cleaner to see if you can increase the friction in them