Have your gear levers seemingly given up the ghost? You’re stuck in the big sprocket at the back, or the big chainring at the front? Or you can shift to some of your gears but can’t get to the rest? You can shift gear if you move the lever extremely slowly, but not at a normal pace? Or you’ve pulled that bike out from where it’s been under the house for a couple of years and the levers don't work any more?
Modern indexed shift levers have some fairly intricate internals including small pawls (small bits that push another part by engaging in grooves or valleys in that other part, as in a freewheel) that carry out their task with the assistance of very small springs. The shifters are lubed at the time of manufacture, and, generally, never again. The problem that arises in time is not a lack of lubrication, but that the original factory lube thickens to the point that it slows or prevents the movement of these light springs and pawls.
The proper (very proper) fix for this is to remove the cable from the shifter, remove the covers also, and disassemble the shift mechanism from the handlebar bracket (or brake lever) and to degrease it thoroughly, rinse it well, dry it well, then re-lube and reassemble.
However, the easy (and about 95% successful) fix is to spray a good thin lube (we use Tri-Flow) into the shifter from wherever you can manage. The port for cable installation is usually the easiest, and will often have a plastic plug about 8mm across that unscrews to allow access for the cable. Sometimes a plastic cover can be easily removed to allow better access. Not fixed yet? If you have access to a compressor, blow the new lube back out with an air gun. Then repeat the lube and air-blast a few more times.
Still not right? I had a lever today that was pretty stubborn so I removed one of the plastic covers and turned the bike upside down so I could see well into the lever. I identified the sticking pawl and with a small screwdriver moved it back and forth gently for a bit to loosen the factory grease. Then, lubed and air blasted (with good aim now) and worked the lever for a while until it worked as well as the day it was made.
The beauty of the easy fix is that if the problem isn’t perfectly fixed, the way to improve it over time is simply to spray another dose of lube into the lever each couple of weeks, using the bike in the meantime. Easy!