Practice sight-reading on your own. The most important aspect of successful sight-reading is gaining familiarity with your instrument and music and improving this skill in the practice room. All the time you spend reading music will help improve your reading ability, but for best results, practice sight-reading in a manner that replicates sight-reading in rehearsal and performance. Choose music that is unfamiliar to you, but somewhat easy for your skill level and practice reading it through with a metronome. Discipline yourself not to stop and correct mistakes. When you have mastered sight-reading easy music, move into sight-reading something harder on a regular basic.
Scan your page before you start to play. Observe your key and time signatures, then look for anything tricky that may throw you off. This can include key, clef and time signature changes, unexpected accidentals, tricky rhythms or tough technical passages. Knowing where these things occur before you start to play will make you better prepared to read through without getting lost or making mistakes.
Keep your eyes ahead. Whatever measure you are currently playing, your eyes should be watching the measure ahead of it. Though this skill can take some time to master, being able to watch what is coming up will give you an enormous edge in terms of preparing your fingers to move quickly. This is especially important if playing instruments that require a wider range of physical motion, such as trombones, percussion instruments and keyboards.
Focus on tempo and rhythm above all else. Strive for the greatest accuracy you can, but remember that in an ensemble setting, it is more important that you stay in correct time with your fellow musicians than that you play all of your notes correctly. Likewise, it is more important that you move from measure to measure at the right time than that you play each rhythm within the measures correctly.
Listen. When sight-reading, it is easy to get so caught up in watching your music that you forget to listen to what is going on around you. However, this is a time when it is more important than ever to pay attention to what your fellow musicians are doing and to adjust your playing accordingly. Being well tuned in to the other members of the ensemble can also help you get through tricky passages together.
Watch. If you have a reliable conductor, pay attention to what she is doing and be prepared to change according to her signals.