If its a weak deraileur spring its best to replace the deraileur. A cheap one will do if your on a tight budget. I replaced a nice looking Shimano 600 rear derialeur with a cheap SunRace years ago thinking that I would wait a little and get another 600. Wow... The cheap on is still on there going strong.

As far as the shifters are concerned, I just went through a real tiff trying to rebuild my neighbors old indexed shifters. His problem was intermittent skipping of gears when shifting to a higher gear. I found the problem to be one tiny tooth knocked of of a hard plastic gear/cam. It was not fixable.

My experience is with old road bikes. When I see the abuse gears, chains, and indexed shifters go through on mountain bike, I wonder how they last more than a few months if any...