Practice the lines, spaces and key signatures if you feel shaky on these basics. Use flash cards or note spellers for practice. Note spellers are workbooks that give you a chance to practice writing and naming the notes on the staff. Work also on the ledger lines, those little lines above and below the staff, if you use them for your instrument.
Work on theory, especially the chords, arpeggios and chord progressions. Study with a theory book, and practice singing or playing the arpeggios. Examine some of your sheet music, and write in the chords. In instruments that play only one note at a time, write the chords that the melody outlines or implies. Pedagogy expert Barbara Fast says that knowledge of theory helps sight-reading.
Practice to improve your technique. You need technical proficiency to sing or play music at a proper tempo at sight. Work especially on the patterns that occur often, such as scales and chords.
Look at the music before you actually sing or play to determine the tempo and key signature. Also read the music through and think it in your head, tapping the rhythm.
Set a metronome to a tempo you can hold and begin to sight-read. Alternately, you can tap your foot to an even tempo. Read straight through the music without stopping, hesitating, or making corrections. Force your eye to keep moving.
Practice sight reading every day for at least 10 to 15 minutes, gradually increasing the difficulty of your music. Include music in a variety of styles, tempos and keys in your sight-reading. For example, if you normally play classical, practice with popular music or show tunes as well. You will become proficient by reading many types of music.
Look for opportunities to sing or play with others in groups or ensembles. Working with others will give you enjoyable bonus practice. You will have to hold a steady tempo and not backtrack to stay with others. If you practice sight-reading regularly, both alone and with others, soon you will be reading sheet music much faster.