The ability to easily sightread music is like our ability to read these words. It gives us the access to an entire world of music that's literally at our fingertips. But most pianists struggle with this important skill.
Here are 5 ways to become a fluent sightreader:
1. Play Bach's 4-Part Chorales
When I was in high school, I spent about a year reading through the Bach 4-Part Chorales for 45 minutes every day. Give it a try. They will helped you visually "take in" vertical structures very easily.
2. Keeping a steady tempo vs. slowing down at times
Practice this both ways. Sometimes it's good to do as everyone says and keep the tempo going even if you make mistakes. At other times, though, it's helpful to freely slow down when you get to a challenging passage, to get it right. The key is to decide in advance if you're going to do this. If you decide to stay in tempo, then stay in tempo no matter what happens.
3. Play for musical theater auditions
Playing for musical theater auditions was what really made me a good sight reader. But even then I couldn't sightread all the notes on every piece. So I got really good at improvising something that was just as good or better than the printed music, in the same style of course. Putting yourself in the position of playing for someone's audition will make your sightreading brain and intuition work better.
4. Sightread with a partner
If you know a classical instrumentalist who plays clarinet or another instrument, see if they want to spend an hour each week reading through repertoire. We get better at sightreading when we're doing it with another musician.
5. Learn your chords
Learn all your chords and become able to recognize which chords you're playing at all times, even when there are only notes on the page with no chord symbols. This will enable you to scan each measure for the chord tones, which give you a quicker sense of what's going on musically as you play through the piece.
Practice sightreading using these approaches for a year or so, and then you'll enjoy the fruits of your labors for a long, long time.
Enjoy the journey and "let the music flow!"