Ideally the timber should be quarter sawn. Imagine a board taken right through the middle section of a tree (discarding the heartwood). The annular rings run vertically, and any movement (distortion) will be minimalised - it will shrink or expand in a much more regular manner, keeping its rectangular shape.
By comparison a board cut ¡®through and through¡¯ is much more likely to distort. The wood in Kevin¡¯s photo looks close to quartered, good regular ¡®rings¡¯ on the surface instead of a wide, chaotic figure (rings not hitting the surface at 90 degrees). A bit of Googling will explain it better than I can.
The other thing is to obtain timber that has been properly dried, rather than some intended for building work (and thus just air-dried). You could look at recycling (¡®upcycling¡¯?) a pine chest of drawers or other item of furniture maybe. You would know that it was properly dry.
I¡¯m in the UK so things will be different here. Years ago there seemed to be a much wider range of softwoods available. Shelves were often made of ¡®Piranha Pine¡¯, and you could buy the incredibly stable Yellow Pine used in pattern making.
Ply does have the benefit of consistency (usually)!
