There is no skill more important or more telling of one’s true abilities than sight-reading. True fluency is reflected in the ability to read anything on the spot. As mentioned elsewhere, there are two main factors that will contribute to sight-reading.
Being in control
Obviously the better you play an instrument (technically) the easier it will be for you to read something well. It also makes sense that the more practice you have at reading pieces and reading various key signatures, rhythms, etc. the better you will be able to read a novel piece of music. You become more aware of what to expect. Reading is a skill that you have to practice.
Tips on how to improve at sight reading
1. Read new pieces with a metronome- Metronomes give you no wiggle room with time. You have to keep going when the beat is unwavering. No matter how 'bad' your performance gets, keep going.
2. Play duets with anyone. Always have a duet book, and always be willing to read out of someone else’s duet book!
Schedule sight-reading time in your practice plan. Have an etude book that is purely for sight-reading.
3. Figure out the goal for YOU based on the level of difficulty. Sight-reading pieces that are considered too easy or too difficult can still be productive if you approach them in the right way. In general, the easier the piece, the higher the level of musicianship you should bring to your performance.
For 'easy' pieces- focus on consistency. Did every articulated note have the same brilliant start? Were your phrases controlled and consistent?
For 'difficult' pieces- focus on survival. Play downbeats and skip what you need to to keep your place. Try to 'catch' more and more as you develop fluency.
4. ACT- do not react. Practice the art of letting errors go by and staying 'in the moment'. Focus on accurate big beats and don’t get caught up reacting to mistakes (which causes more errors). You have to practice the art of continuing, even when you want to stop and figure out what went wrong.
5. Learn your strengths and weaknesses. Do you struggle with rhythms? Key signatures? Odd meter? Accidentals? Know what your sight-reading weaknesses are and prioritize them. Get together with people that are strong at your weakness.
6. Read in all 12 keys (or more). Read in less familiar meters. Read in less familiar clefs. Get out of your comfort zone.
7. Absolutely no judgment- just read, and forget what happened. Move on.
Here are some links for music to sight read: