-In 1994, Shimano introduced a new derailleur called the "Light Action 
 Derailleur".  This system has a lighter return spring in it, making the 
 shifting much mellower when paired with Shimano's own shifters.  When 
 paired with Sram's Grip Shifts, shifting with the Light Action rear 
 derailleurs can be very troublesome.  The light action spring might not 
 be strong enough for someone to upshift when the bike is not in top 
 shape.  There are a few ways to fix this problem
-Install a Bass Worm.  It's a rubber tubing that seals the cable end and 
 helps the cable to return.  
-Install high-end cables.  Gore-tex Ride-on cables are teflon lined 
 cables and cable housings that greatly reduce friction and help the cable 
 to return.  Slick Whip is a lower price version of these cables.
-Lube your cable with a very dry lube; this should reduce friction (do not 
 do this with Goretex Ride-on cables.)
-Install a Power Spring.  It replaces the light action spring with a much 
 stronger spring.  Your shifting should improve a lot, but it is a pain to 
 install for some.
-Get a pre-Light Action derailleur.  Any Suntour derailleur or pre-1994 
 Shimano derailleurs should do well.

Some added:
John Stevenson [johnstev@world.net]
-Give up using inferior shifters and switch to RF+ :-)

Shimano's 1996 line-up includes a long-arm version of the 105 rear
derailleur with a non-Light Action type spring. This may also help solve
this problem, though the track record of 105 mechs on MTBs is poor as far
as durability is concerned.