So your bike isn't shifting properly? OK. Come in, sit down, we can talk about this.
Now, now, tell Uncle Knuckler everything. Oh, so it skips around. It takes two clicks to get it to shift. Anything else? It's driving you crazy. Last question, when was the last time you changed your cables/housing? Six years ago? Holy shit, let's start there.
There are three parts to most shifting systems: (the exceptions are Di2 and that new Syncros stuff) the shifter, the cables/housing and the derailleurs. The shifter is where your input is applied. Want a harder gear? You hit the button releasing or pulling cable tension. The derailleur receives that input via cables/housing and moves the direction that you wanted it to. It's a simple system.
The weak link to the system is the cable/housing. Over time grit and shit will enter the housing degrading the inner sleeve causing friction. The outer sleeve of the housing will also wear exposing the inner radials adding flex to your system. Sure you can buy that fancy Gore stuff to extend the life of your cables but one thing is certain, your shifting performance will degrade over time, regardless of what you do. Yes, I said regardless of what you do, at some point your bike is not going to shift like it should. Big fucking deal, get over it and buy some new cables/housing. How often? Well, if you need me to give you an exact time frame, once or twice a year. If you think you can handle a little more freedom, change them when your shifting isn't working perfectly.