Are these the old "thumb" shifters? Nearly all shifting problems are related to the actual cables and cable housing coming out of the shifters....not the shifters themselves. Since they worked fine and have slowly stopped working due to nothing catastrophic, I would strongly suspect the cables/housing.

Replacing them is fairly inexpensive and is the single best thing you can do!

If you're handy read this:

http://www.sheldonbrown.com/cables.html