Recently I was practicing piano and my choral director walked by and tapped on the door and told me that, when he grew up, he wished he did a bit of sight reading every day when he was young. Hence my question: what is a good short (10-20) minute sight reading exercise I can do at the beginning of each practice session? Is 15 minutes too short for an effective sight reading practice?
This borders on being opinion-based---we all probably have our own preferences for sight reading practice based on our own individual strengths and weaknesses---but a common strategy is to read hymns.
Hymns are nice because they tend to be block chords, the melody is clear (and often recognizable), and the four-voice texture is a good starting point.
Spend a minute or so looking through the hymn, check out possible difficult spots, and sight read it. If you can, try reading through it a second time to fix what you missed. Go to the next hymn. Lather, rinse, repeat until your 10--20 minutes are up. (No, I don't think 15 minutes is too short; it's more than most do!)
Furthermore, you can continue using the same hymns as you improve by a) transposing the hymns at sight, b) embellishing various aspects of the hymns, c) improvising progressions that modulate from the key of Hymn A to the key of Hymn B, and so on.
For what it's worth, I think the Lutheran hymnal has the best repertoire, but that's 100% opinion!
If your library has a music section, borrow books of piano music and sight-read the pieces. Piano arrangements of modern popular songs are usually simple to read. Your choral director probably has a lot of anthems. Sight-read the voice parts, and then sight-read the piano parts. Start with anthems with the voice parts written on two staves, and move on to those with the voice parts on four staves.
I recommend using the improve your sight reading series by Paul Harris. It is an excellent series with progressive studies and exercises for tackling gradually increasing sights reading challenges.
A big problem that I had when learning was the advice was basically ‘read a lot’. This book introduces things in a very systematic and thorough way. I use it with all my students.