Reading anything and everything you can is the best way to do it. Alone and in ensembles. It's that easy. :lol I've never used any sightreading books because there are so many method books, solo literature, etudes, etc. that you can use those for sightreading practice.

I've also heard that learning to juggle helps (really!), because it helps you improve your sense of the big picture as opposed to focusing so narrowly as some of us do at times when sightreading.

Piano players are used to looking at multiple staves and looking ahead, so I'd imagine that's why they tend to be pretty good.

My $0.02.

Michael