Every collaborative artist recognizes the importance of sight reading in our profession. Throughout my piano study, most teachers claimed you either could sight read or you couldn't; they held there was little you could do to improve your skills in this area. Since that time, I have discovered through my personal experience and those of other piano students that sight reading CAN improve.
For the working collaborative pianist, my suggestion is to read new solo literature on a regular basis. The repertoire for solo piano is abundant and of varying technical demands. Personally, I begin with the sonatas of Haydn, Mozart, and Beethoven. There is no pressure to play the works at tempo; accuracy is the goal, so play at a tempo that you can handle. As a student, I was instructed to sight read daily for thirty minutes; now I simply strive to do some amount of reading each day. Of course, reading repertoire from the vocal and instrumental literature might be an option for some pianists. I avoid this music so I won't be tempted to feel the necessity of learning the works and because solo literature is more readily available to me.
When do I introduce sight reading to my own students? As early as possible! By having them reading new repertoire regularly and often, students are not aware that they are developing a desirable skill. Once a student begins sight reading well enough to be independent in the activity, I encourage them to explore all types of music: hymns, solo pieces, movie soundtracks, and pop songs. I basically make a game of it. I want to see how many songs they can play through before our next lesson (remembering that the goal is not learning it or having it ready for public performance). The student keeps a list of the songs they have read and brings some of their favorites the following week for us to look at together. This allows me to praise them for their hard work and applaud their willingness to challenge themselves with difficult music while providing insight into their interests. The results have been tremendous; surprisingly, many of the students ask to repeat the exercise again and I gladly oblige!