Here's the traditional advice on sight reading first. Read on for unexpected methods to improve this skill.
Take a moment to scan the music score before you begin playing and ask these questions:
1) Figure out the key. Remember that it may not be in a major key!
2) Notice the time signature. Prepare to "count in" in the appropriate meter. Not everything is in 4/4 time.
3) Scan the piece for accidentals. How common are accidentals throughout the piece? Do you see sharps and flats?
4) Scan the piece for unexpected chords. For instruments that play chords, like the guitar or piano, notice if there are any stacks of notes that you don't recognize. Take a moment to calmly decipher the chord.
5) Notice the rhythms. Are they consistent throughout, ie. straight eighths, Bossa Nova, the Charleston, or some other repeating pattern. Take a moment to calmly find...