Why practice it
Sight-reading is playing music you have never seen before. It comes up in chair tests, auditions, and the sight-reading room at festival, and it is also how fast you learn every new piece. Like scales, it gets better with a few minutes a day, more than with one long session.
Read before you play
Take the time you are given. Check the clef, the key signature, the time signature, and the tempo, then look for the hardest measure and the accidentals. Count the rhythms in your head and finger or stick through the hard spots silently. The sight-reading checklist walks through it step by step.
Keep going
Once you start, don't stop to fix a mistake: keep the beat and get back in at the next note you can. A steady pulse with a few missed notes sounds better, and scores better, than the right notes with stops in between. Pick a tempo slow enough to make it to the end.